Bettyboo. I think the easiest one to do, especially if the children are quite small, is a rabbit. You could use the paper cup upside down, and cover it with cottonwool, to form the body, then cut of the bottom one-third of another cup, cover again with the cottonwool and stick it on top of the body, to form a head, then tease out the ears from the head. Eyes, and nose can be done with felt tip pen. If you have some ribbon, you could tie a bow round the finished rabbit's neck.
You could also cut out a carrot shape from white card, and the little ones could colour it in orange for the main part and green for the tuft of leaves. Hope this helps. Schutz.