The first thing that comes to mind is, "Why God?" Then we say to ourselves, "what do we say to the children?"
Well, first we must let the children know that this world consists of both good and evil people and this goes back to The Garden of Eden.
We must always try to draw people over to the good side so that the righteous may prevail We must ask God that when the bad things happen to the good people, that they didn't feel the pain and suffering.
We must also let them know that the children are now safe in heaven from all harm and danger because He says in Matthew 19:14, "Let the children come to me and do not hinder them. For the Kingdom of Heaven belongs to such as these.
We also need to have the children thank God that they still have life on Earth and a chance to make a difference. Ask them to always do something in their lives to make the little angels proud that their departure was not in vein. Also ask them to ask God to comfort those who morn as he said he would in Matthew 5:4.
Then tell them to tell God that when the time comes for them to join Him, that they would love to meet the 20 little angels that we surely know are in heaven.