Stupid kids won't stay off of my property...?

We bought and moved into our house after it had been on the market for two years, just enough time for these little kids to get used to playing around in the yard. Well it is our yard now and we would like them to stop tresping onto it. We have spoken to the 'parents', as if irresponsible, non-observant retards are the definition of parent, but the kids continue to come onto our property. It seems as though every day at some point we see them in our yard and have to go outside and yell at them to go home. We have planted hedges along the property lines but they will take some time to grow out. Also, I have a strong suspicion that they have caused destruction to our garden, as their are small footprints and crushed plants, but I can't prove that it was them. What can the law do if I were to call them? I really do not want to be held liable for any accidents that may happen on my property, and I am getting sick of wasting my breath yelling.