My DS does this every day too. When I sat down and talked to him, he said that some of the other children hit him and were taking his toys. So, we have been working on strategies with him to stand up to them.

The carers did not take me seriously at all but eventually I became more firm about one of the children and they admitted that they knew he was an issue (I think that other parents have complained too) and that they have been trying to address with his parents.

It's so hard for them - they are so young to have the resources to deal with this yet. The best you can do is talk to the centre staff until they do something about it.