*hugs* What ifs and guilt are all part of grief. But try to draw comfort from the fact that you did the very best you could, with limited information. You followed the vets advice and did the right thing at that moment. No one can know whether or not he was bleeding, or even if it was VHD unless he has a necropsy, which probably won't actually help anyone.
Try to focus on the other bits, like the fact you were with him all the way through, that you gave him a cracking life, that you did your very best for him.
Yes, kids do deal with things differently, but remember that they won't have shared the bond you did with him, and also that each and every single person does grieve differently. When I was a child, they were 'just pets', but now, as an adult, they are friends.
I found having other bunnies around a real help after my various griefs, because everyone needs friends, and that's what they were.
Also, something to consider might be to make a 'Milly box' with all things that remind you of him, so that when you miss him, you can look in the box and remind yourself of the good times.