What are the rulings on keeping stuffed toys like those of Hello kitty? Are they considered images? If so, would you have to throw them out (since donating them or giving them away would mean someone else gets those images), or could you just cover them before praying Salah?

I do not know if hello kitty resembles an actual cat or not. Some of the people of knowledge permitted children's toys, but avoiding them is safer and is what is advised. So rather than keeping them, the better path is to bring the child other toys that are free of images, for whenever one door is closed, another good one can be opened.