I don't see why you can't tell them to get off your property, but I would do it politely and firmly in case any of them have parents you'd rather not get hassle from. I have a young daughter who plays with our neighbour's grandson, recently they have attracted a posse of slightly older children who congregate around the path in front of our house and sometimes find their way on our property. I tell them firmly not to muck about in my front garden and to stay out, and they duly oblige, we have a fence and gate which makes it easier (our neighbours aren't so lucky) - keeps them out and they can't sit on the fence either, which would also annoy me. I have a bolt on the inside of the gate which I sometimes use in the evening (it's very stiff so even if they wanted to open it by leaning over, they'd have a hard job) - that keeps them away much better if they are irritating me, and they understand it's off limits.