That guilt is a tricky emotion to manage, especially when children feel happy in their new life. It's heartbreaking for everyone involved, and you're right - age-appropriate resources are so important in helping them understand and process these complex emotions.
Using books to initiate discussion is a clever strategy! I think having a visible collection of such resources is beneficial, too, so kids can gravitate towards them when they need to. That normalizes the situation slightly, showing that these aren't taboo topics but rather common - and manageable - struggles that other kids have gone through as well.
The counseling route is one I've also found indispensable, mostly because the professional can help draw out the child's feelings and thoughts in a safe, guided way. Plus, having an unbiased third party in the room can make the talks easier for everyone!
It's heartwarming to hear you've found a network of similar families. There's really something to be said about shared experiences - it's a unique kind of support that not many outside the situation would understand.