I really don't see the issue with two 10-12 year old girls on your dock. They were probably just bored and exploring a bit, or saw a fish etc in the water. If they weren't hurting anything, and you could see them why not watch for a few minutes and if you noticed them making mischief, or picking flowers then say something. If they are neighborhood kids you just set yourself up for a world of trouble, and trust me word goes quick through all the kids. You have gained a reputation and if they are going to do anything "bad" to a neighbor, they're probably going to do it to you. Speaking from a bit of experience (we had several little hellions on our street) the kids don't target the people who are kinda laid back, don't get mad if a kid walks through their yard, they target the people who moving a couple of rocks, or ding dong ditching etc. is going to get a rise out of them.