Stealing is unacceptable. At 7 years old, he knows better. If you can, advertise and replace this family. The parents are obviously not doing enough to quell this behavior. If you cannot afford to lose them, or cannot fill the spot, I would pat them down every single evening before pick up. I also would not allow any backpacks/diaper bags that are accessible to the daycare kids.

Your poor son does not deserve his beloved toys being taken. It's hard enough on our OWN children when doing daycare (having to share their personal space, mom, toys, etc) but to add stealing to the mix. Seems like he and the 3 yr old dcb enjoy playing with his special toys together. Perhaps only allow supervised play only with these special toys. You also need to keep a third eye on the 7 yr old.