So sad!
You clearly did all you could for the kitten. I have held an animal while it died on a couple of occasions in my life as an adult.
I don't think I could have kept it together as a child. What a strong little man you guys are raising!Sorry for your loss. It's never easy letting go of a pet no matter how long you've had them. It's even harder to do with kids.
Now you'll have to spoil the other kitten twice as much.