Wearing Masks or Costumes of Animals at Children’s Parties?
Question

What is the ruling on wearing a mask or costume in the shape of a human or animal, such as a bear and so on, to take part in children’s programs and make the children have fun?
Praise be to Allah.Many scholars have granted concessions with regard to children’s toys, even if they are in the shape of an animate being, because of the report narrated by Abu Dawood (4932) from ‘Aa’ishah (may Allah be pleased with her) who said: The Messenger of Allaah (peace and blessings of Allah be upon him) came back from the campaign to Tabook or Khaybar and there was a curtain over her niche. The wind lifted the edge of the curtain and uncovered ‘Aa’ishah’s toy dolls. He said: “What is this, O ‘Aa’ishah?” She said: My dolls. He saw among them a horse with two wings made of cloth and he said: “What is this that I see in the midst of them?” She said: A horse. He said: “What is this that I see on it?” I said: Two wings. He said: “A horse with wings?” She said: Have you not heard that Sulaymaan had horses with wings? She said: And the Messenger of Allaah (peace and blessings of Allah be upon him) smiled so broadly that I saw his eyeteeth. 
And al-Bukhaari (5779) and Muslim (2440) narrated that ‘Aa’ishah (may Allah be pleased with her) said: I used to play with dolls in the house of the Messenger of Allah (peace and blessings of Allah be upon him) and I had friends who would play with me…” 
This is with regard to toys and costumes, and there is a well-known difference of opinion concerning this matter. Some of the scholars forbade children’s toys if they are in the form of an animal such as a lion or tiger and the like, and some granted a concession allowing that. 
See: Ahkaam al-Tasweer fi’l-Fiqh al-Islami, p. 241-261; Fataawa Noor ‘ala al-Darb by Shaykh Ibn ‘Uthaymeen, tape no. 374, side A. 
According to the view that costumes are permissible in general, if a person wears something that is made in the shape of a bear, for example, and he tries to imitate the way it moves and walks, this includes two things concerning which there are reservations: imitating animals and wearing an image or something on which there is an image. 
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said concerning contact lenses: 
If they resemble an animal’s eyes, such as those look like the eyes of a cat or a rabbit or any other animal, that is not permissible, because resembling animals is not mentioned in the Qur’aan and Sunnah except in the context of blame. Allah says (interpretation of the meaning):
“And recite (O Muhammad صلى الله عليه وسلم) to them the story of him to whom We gave Our Ayaat (proofs, evidences, verses, lessons, signs, revelations, etc.), but he threw them away; so Shaytaan (Satan) followed him up, and he became of those who went astray.
And had We willed, We would surely, have elevated him therewith, but he clung to the earth and followed his own vain desire. So his parable is the parable of a dog: if you drive him away, he lolls his tongue out, or if you leave him alone, he (still) lolls his tongue out. Such is the parable of the people who reject Our Ayaat (proofs, evidences, verses, lessons, signs, revelations, etc.). So relate the stories, perhaps they may reflect.
Evil is the parable of the people who rejected Our Ayaat (proofs, evidences, verses and signs, etc.), and used to wrong their ownselves”
[al-A ‘raaf 7:175-177]. 
And He says (interpretation of the meaning):
“The likeness of those who were entrusted with the (obligation of the) Tawraat (Torah) (i.e. to obey its commandments and to practise its laws), but who subsequently failed in those (obligations), is as the likeness of a donkey which carries huge burdens of books (but understands nothing from them). How bad is the example of people who deny the Ayaat (proofs, evidences, verses, signs, revelations, etc.) of Allaah. And Allaah guides not the people who are Zaalimoon (polytheists, wrongdoers, disbelievers)”
[al-Jumu ‘ah 62:5]. 
And the Prophet (blessings and peace of Allah be upon him) said: “The one who takes back his gift is like the dog that vomits, then returns to its vomit.” And he said: “The bad likeness is not for us.” And he (blessings and peace of Allah be upon him) said: “The one who speaks on Friday when the imam is delivering the khutbah is like a donkey that carries books.” 
End quote from Fataawa Noor ‘ala al-Darb. 
See also Majmoo‘ al-Fataawa by Shaykh al-Islam Ibn Taymiyah (32/256-260), where there is a detailed fatwa about the prohibition on resembling animals or imitating their movements or sounds. 
Shaykh Dr. Ahmad ibn Muhammad al-Khudayri, a member of the Faculty of Imam Muhammad ibn Sa‘ood Islamic University (may Allah preserve him) was asked:
We are a group who work in children’s theatre and we offer useful programs in the park or in a theatre for children, where children and their parents come to watch us. We segregate men from women and the children sit in the chairs at the front. Some of the young people wear costumes in the form of a fox or bear or cucumber or orange. Our question is: is acting and wearing these costumes permissible? 
He replied: If what you are offering in the theatre is a useful program aimed at benefiting children, and there is no mixing between men and women in that place, then there is nothing wrong with this action. But resembling animals is not something appropriate for the Muslim to do, because it is humiliating a soul that Allah has honoured with reason, and resembling animals is not mentioned in the Qur’aan or Sunnah except in the context of blame. If the costume of this animal is three-dimensional, it is not permissible because it comes under the heading of making images, which is forbidden in the texts. As for costumes resembling inanimate objects, there is nothing wrong with wearing them and using them. 
From the Islam Today website. 
It is not permissible to wear anything on which there is an image of an animal or human. 
It says in Mataalib Ooli’l-Nuha (1/353): 
It is haram (impermissible) for both males and females to wear anything on which there is an image of an animal, because of the hadeeth of Abu Talhah who said: I heard the Messenger of Allah (blessings and peace of Allah be upon him) say: “The angels do not enter a house in which there is an image or a dog.” (Agreed upon) 
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked about the ruling on wearing clothes on which there is an image of an animal or human. He replied: It is not permissible for a person to wear a garment on which there is an image of an animal or a human. It is also not permissible for him to wear a headcover or the like on which there is an image of a human or an animal, because it is proven that the Prophet (blessings and peace of Allah be upon him) said: “The angels do not enter a house in which there is an image.” End quote. 
What appears to be the case is that wearing something that is made in the shape of an animal is more contrary to sharee‘ah than wearing regular clothes like shirts and so on, on which there is the image of an animal. 
Thus it is clear that there is no concession with regard to what you have mentioned about wearing a costume in the form of a human or animal, even if the aim is to take part in a program for children so that they can have fun. 
And Allah knows best.