The resilience of children is amazing, but if they are old enough to ask questions they need to be answered truthfully*. they will often process things in a very simple way and will offer a route to acceptance of the situation that wouldn’t occur to adults.
my dad died suddenly four years ago, my kids were asleep at the time and trying to explain why grandpa wasn’t there any more was a very hard thing to do.
my thoughts are with you, its a horrible position, but as above, focus on the daughter as she will need help and helping her will help others too.
*obviously spare any gory or upsetting details, but saying someone has gone to a better place can lead to confusion, which is hard to unpick.