friend at work lost his father recently and I had the same discussion with him, and one thing the bereavement counsellors said to him was don’t use the ‘gone to sleep forever’ line with the kids as that can cause sleep issues.
otherwise I like the body worn out, gone up to heaven / to become an angel (which is not necessarily religious at that age if you don’t want it to be) / to become a star. I like the ‘do you remember what it was like before you were born’ and I also liked the Rev one as per link above.
As for religion / god, I’m not religious but personally don’t have an issue with using that as an explanation if it works at that time. The girls are taught religion at school and in time will make their own decisions on that. I’m happy to explain that some people don’t believe in god, and others like me are not sure.
Sad at your loss.