I am incapable of accuratedly knowing what an atheist would say. Perhaps something like..’be thankful that your child is at peace, or is no longer in pain’...something like that. Although, an atheist may not care how you feel, as they know it is the child that died, and you have to deal with these things in your own best manner.
An athiest would be incapable of saying such a thing without betraying his atheism. As to "no longer in pain" who said anything about pain? What if the child just dropped dead or was hit by a car or drowned in a pool?
Logically with an attitude like that, the Atheist should be walking through hospital wards with a AK-47.