As-salamu alaykum sister,

I am not an Islamic Scholar however I would suggest that insha’Allah you teach your children the beautiful obligation and blessings of Ramadan in a way that is comprehensive, yet in a way that they can understand and reiterate to their schoolmates. Perhaps you can ask them what are the most frequent questions they are asked, and go over simple answers which they can readily respond with. Have them practice with you. Ask the some questions and listen to their responses as if it were a role play. Also, if they are asked a question that is too in depth for them, it is okay for them to say they do not have the answer but they will ask and give them the answer tomorrow. This is also a great way to teach your children the preparations and benefits of giving dawah. Regarding fasting, while the usual age for fasting is when a child reaches puberty, but if the younger ones want to try fasting alhumdulillah let them try! It is a blessing.