Spiritual Training of Children
Question

I have a young child who is three years old, and I want to instill faith in his heart. What should I do?
Praise be to Allah.A child who is three years old should see his mother and father praying, and he should hear them reciting Quran, for if a child hears his parents and brothers reciting Quran and daily dhikr (words of remembrance) repeatedly, this nourishes his soul and brings his heart to life as the rain brings life to a dry barren land, because when a child hears his parents remembering Allah and sees them worshipping him, that has an effect on his own words and actions. 
An example of that is the following story of a young girl: 
Her mother finished her Wudu, and her three-year-old daughter washed her face and hands, copying her mother, then she raised her forefinger saying, “Laa ilaaha ill-Allah.” This indicates that the little girl had been watching her mother and noticed that there was a specific dhikr to be said after making Wudu. 
In another story, a mother did the Sunnah of Wudu (saying “Laa ilaaha ill-Allah”) one day, then she got up to finish her housework. Her daughter had gotten used to seeing her mother sitting after praying until she had finished reciting the du’aa’s (supplications) to be said after prayer, and she noticed that her mother had got up straight after doing the Sunnah prayer, so she said to her, “Why did you get up before saying, “Astaghfir-Allah”? This attitude indicates how closely children watch their parents. 
Man is exposed to sickness and diseases, and a child may fall ill. Then his illness should be an opportunity to strengthen his relationship with Allah, by reminding him of the virtues of good health, and that it is the blessing of Allah, and that we must thank Him for that, and that man has no power and no strength except with his Lord. When giving him medicine or going to the hospital, we should tell him that healing comes from Allah, but these are means which Allah has enjoined upon us. Then we should get them accustomed to ruqyah (incantations)as prescribed in sharee’ah. We should tell them about the example of the Prophets and how they took the necessary means and put their trust in Allah, such as the story of Ayyoob (peace be upon him) and his sickness, and the story of Ya’qoob (peace be upon him), when he told his sons to enter by different gates, and that would not avail them against Allah at all, and how he left the matter to Allah. Allah tells us that Ya’qoob said: 
“Do not enter by one gate, but enter by different gates, and I cannot avail you against Allah at all”
[Yoosuf 12:67 – interpretation of the meaning]
One of the most important things is to remind children to seek reward and to bear sickness and medical treatment with patience. One little girl reminded her mother that Allah wanted her to bear with patience the sickness which the doctors described as chronic. That was according to human doctors, but healing is in the hand of Allah. The mother remembered that this child had to take medicine twice every day, and her mother always reminded her to seek reward. One day this little girl said to her mother, “I will get reward because I am taking this medicine.” She said that as if she felt proud of this reward and the reward that her family and siblings would receive.