Do they let you join in? If not think about small steps to letting you join in e.g. could you play with something similar in a similar way alongside them rather than trying to take the things they have? Or it may be just being nearby, tuning into their actions and sound and copying some of these. It may be that you have to start quite far away and gradually get a bit closer until they let you into play. Or it may be they are more than happy for you to be there in play, but not want you to play with the items.