Hi Simon, this sounds awful for all of you. Huge sympathies. Does Jess have any adult role models/mentors that she chats to? (School teacher/music teacher/sports coach/drama coach, etc.) I bet you've been a great dad throughout the whole thing so far, and I'm sure you've probably already kept her school completely in the loop.
They'll both be finding it hard, and friends may not know how best to suport them. I've never been through it myself, but my friend's daughters recently lost their dad, (they're 12 and 14). They have said that their school counsellor and class teacher have helped hugely (so there's someone at school who understands, and that they can chat to confidentially. It's someone 'outside' that can't be hurt or upset by anything they say.) They've also said it's much better if, when they see someone socially, that the loss is acknowledged in some way, if not particularly dwelled on. I think if there's always someone around them who is aware of their loss, but doesn't make them focus on it it seems to help. Flexibility and understanding of those around them seems to be key.
It might be some kind of consolation for Jess to have a role within the household to focus on at the moment. Any female friends/relatives who can step in, say, at weekends so you all have companionship and support?
The kids are lucky to have such a switched-on dad. I'm sure you already know about Cruse here. I don't have experience of them, but my closest friend is a Samaritan and she really rates them.
Lots of love and luck to all 3 of you.