When scientists say “the universe” in the context of the big bang, it is usually shorthand for the temporal-spacetime envelope that we’ve been able to observe and make inferences within, the observational limits being traceable to that era.

Scientific evidence has not established that nothing existed before the big bang.

Children are perfectly capable of understanding that there are limits to what we have been able to obtain data about, so they don’t really need us to mix scientific understanding with simplifying dogma.