Dealing With Children Who are Heedless About Prayer
Question

I want to make my children keen to pray, because they will pray once then forget about prayer many times. I always advise them and pray to Allah to guide them. How can I encourage them and make them love prayer?.
Praise be to Allah.Firstly: 
There is no doubt that prayer is very important and is the most important matter in Islam, as it is the  foundation of religion, the means of success and a sign of piety (taqwa). It is the first thing for which a person will be brought to account on the Day of Resurrection: if it is sound, then all his deeds will be sound, but if it is lacking, then all his deeds will be lacking. 
Undoubtedly doing one’s utmost to give a child a sound Islamic upbringing and teach him to establish prayer and fear Allah in word and deed, is a sign of success and being guided.  
Allah, may He be glorified and exalted, said to His Messenger (blessings and peace of Allah be upon him) “And enjoin As‑Salaah (the prayer) on your family, and be patient in offering them [i.e. the Salaah (prayers)]” [Ta-Ha 20:132 -- interpretation of the meaning]. 
i.e., urge your family to pray and encourage them to offer both obligatory and naafil (voluntary) prayers. The command to do something is also a command to do everything without which that thing cannot be done, so it is a command to teach them what makes prayer valid or invalid, and what makes it complete or perfect. 
Tafseer al-Sa‘di, p. 517 
And Allah says of His Prophet Ismaa‘eel (peace be upon him):
“And he used to enjoin on his family and his people As‑Salaah (the prayers) and the Zakât, and his Lord was pleased with him”
[Maryam 19:55 -- interpretation of the meaning]. 
And Allaah says to those who believe (interpretation of the meaning):
“O you who believe! Ward off yourselves and your families against a Fire (Hell) whose fuel is men and stones, over which are (appointed) angels stern (and) severe…”
[al-Tahreem 66:6]
i.e., enjoin them to do good and forbid them to do evil, and do not neglect them lest they be consumed by the Fire on the Day of Resurrection. 
Tafseer Ibn Katheer, 5/240 
Abu Dawood (495) and Ahmad (6650) narrated from ‘Amr ibn Shu‘ayb from his father, that his grandfather said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Instruct your children to pray when they are seven years old, and smack them if they do not do it when they are ten, and separate them in their beds.” Classed as saheeh (authentic) by al-Albaani in al-Irwa’ (247). 
Shaykh Ibn Baaz (may Allah have mercy on him) said: 
Pay attention to your family members and do not neglect them, O slave of Allah. You have to strive hard to guide them. Instruct your sons and daughters to pray when they are seven, and smack them if they do not pray when they are ten, a light smack to help them obey Allah and help them get used to praying on time, so that they will adhere to the religion of Allah and know the truth, as it says in the saheeh Sunnah of the Messenger of Allah (blessings and peace of Allah be upon him). 
Majmoo‘ Fataawa Ibn Baaz (6/46). 
Secondly: 
With regard to the means of disciplining children and raising them with the prayer and to respect its importance, it may be summed up as follows: 
-It is essential that there be a real life example, in the parents’ being keen to offer the prayers on time.
-The father should be keen to take his sons with him to pray, and the mother should be keen to tell her daughters to pray regularly at home.
-Explaining the importance of prayer and that it is an important pillar of Islam, without which one’s religious commitment cannot be complete.
-Encouraging them to establish prayer on time, and explaining that Allah has promised Paradise for establishing prayer, as Abu Dawood (425) narrated that ‘Ubaadah ibn al-Saamit said: I bear witness that I heard the Messenger of Allah (blessings and peace of Allah be upon him) say: “Allah, may He be exalted, has enjoined the five daily prayers: whoever does wudoo’ for them properly, prays them on time, bows properly in them and focuses with proper humility (khushoo‘), has a promise from Allah that He will forgive him. And whoever does not do that has no such promise from Allah; if He wills He will forgive him and if He wills, He will punish him.” Classed as saheeh by al-Albaani in Saheeh Abi Dawood.
Whoever would like to have a promise from Allah, let him pray, and whoever does not care to have a promise from Him and to expose himself to His wrath and punishment, let him forsake prayer!
And one may also quote hadeeths which mentioned promises and warnings with regard to prayer.
-Making the most of all available means to give gentle advice, provide books and audios that speak of prayer and explain how important it is.
-Encourage children to keep company with those who pray regularly, whilst instilling positive motives in their minds to compete in establishing prayer and attaining good.
-Offering moral and material support, such as giving gifts, offering words of praise and encouragement and so on.
-Following the example of the Prophet (blessings and peace of Allah be upon him) in means of dealing with the issue of prayer, as mentioned above in the hadeeth of Abu Dawood, such as instructing them to pray when they reach the age of seven, then smacking them (lightly) if they do not pray when they reach the age of ten, whilst paying attention to wisdom with regard to the matter of smacking: using it if it is thought most likely to be of benefit, and only being strict and harsh at the appropriate time.
-Using means such as shunning and forsaking if a child stops praying or neglects prayer; this is a kind of effective punishment that is prescribed in Islam.
-Offering a great deal of supplication (du‘aa’) and beseeching Allah, asking Him to guide them to the straight path and make them pious people who pray regularly. This in fact is one of the greatest means of guiding one’s children, although many people neglect it.
-The parents should never tire of reminding, advising and disciplining, even if the children neglect prayer repeatedly, and they should never despair of their children’s being guided. No one knows when a word will be of benefit or not.
For more information, please see the answers to questions no. 10016, 103420 and 127233. 
And Allah knows best.