K.S.
Some ideas. Have them work on a booklet to send with each animal to their new home that has their likes, dislikes, routines, food, and exercise programs laid out. That way they can feel involved in helping the animal and the new owners make a smooth transition. It will also allow you discussion time to talk about how each animal will have a home where they can get a lot of love and attention.
You could also make memory books together before each pet leaves with pictures of the kids with them so your kids have something to look at and use to talk about the pet after they have left. I don't believe in trying to "protect" kids from pain and disappointment. I believe in honoring their emotions and providing ways for them to express them and giving them support as they mourn the loss of these friends in their lives.
I'm sorry that you have to do this so quickly and abruptly, it must be really tough for all of you.