If the creek is on your property that the children wish to play in and you do not want them on your property, you have every right to keep them off it. I would post "no trespassing" signs prominently on your property for all to see if you desire.You have legitimate concerns that you could be sued for an injury on your property by the trespassing children.I would write a letter to the children's parents explaining the need for them to not have their children on your property. Keep a copy of the letter. If the parents fail to keep their children off your land in the future, contact law enforcement about it pointing out the posted signs and the children will be taken to their parents most likely by the local police officer.If the children still keep going onto your property without permission, consult with a lawyer experienced in real estate about a possible legal action against the children and/or their parents for invading you property rights.Good luck.