I started to first not let toys in the car at all, but then they needed to be occupied so it was then allowed but they had to bring them back in the house and they are not allowed in stores and not to school. The school is great about not letting things come to school. It something is discovered then it has to go back in the book bag. The only issue I have had is them putting the toys in their pocket and I don't realize they are there and they get washed.
Has there been an issue with the toys being taken or given away? Is the teacher getting upset? I guess you can express that you have to have a level of trust with them and if they can show you they can be trusted for say 5 days in a row, they they will get a reward. Then increase the time and the reward.
Good luck and hang in there.
rumbamel