At least she's feeling your concern, comfort, and love. We don't often get to say goodbye to our little critter friends - usually happens when we're not looking.
Bummer. Can you set up a little intensive care unit with your son for the critter? If he feels like he did everything he could to make it comfortable, it may help. Heating pad on 1/2 a shoebox, fluffy bedding, a favorite treat, etc.
I've been through that kind of thing many times with my (now...