Praise be to Allah.
If a child asks about the Shi’ah, he can be told that these are people who are not good, they commit sins and do wrong things, and they slander the companions and wives of our Prophet (peace and blessings of Allaah be upon him), who are the best of people after the Prophets, and we do not like them because of that. If he has seen them on TV touching graves or celebrating ‘Ashoora’, he can be told that what they are doing is not permissible, and we do not worship anyone but Allaah alone; we do not prostrate to any grave or to any of His creation, and we do not strike ourselves or our children. Thus the child will understand that these people are not good, rather they are erring and deviating. The information given to a child should suit his level of understanding.
In order to correct what has happened, it should be said to the child: We are Sunnis, we follow the Sunnah (teachings and way)of the Prophet (peace and blessings of Allaah be upon him), and we do not introduce innovations into our religion. We do not do what these people do. If the child is able to watch some of the wrong actions of the Shi’ah, this should be accompanied by clear warnings about them. You can also look at trustworthy websites that warn against the Shi’ah and explain why they are misguided.
And Allaah knows best.