There is a great children's book called lifetimes. Explains the concept of everything having a lifetime-animals etc - I use it when discussing death with chchildren. I'm on my phone so a bit hard to check for the author. But if you can't find it pm me and I'll will try and find a link for you The library may also have it.

your responses will be most important - Sounds like you are doing good. But if she seems distressed and is dwelling too much then a counsellor may be helpful. Or you could chat to one yourself for some tips on how to handle it.

Im sure with a mum so keen to get it right things will fall into place. Kinda worrying for you having her ask such deep questions, I bet.