Some don't have an answer or don't care. Some parents show all the religious answers to disillusion the uniqueness and originality of major religions' answers. Many children don't ask or care. Many children are perfectly fine with documentaries about the Big Bang, seeing the complexity involved.

I asked, my parents didn't know and cared little to find out, I found documentaries and audiobooks made for children. It took a long time for me to understand that the boring religious stories they subtly taught in school were seeking to at least partially replace the scientific narratives.