I am unsure about playground/pool etiquete especially when there are personal possesions involved. I encourage my daughter(4) to share but I also have to discourage her from playing with other childrens toys if the other kid doesn't want to share. It has to be confusing to her because it's a mixed message. Other complications come in when the children are different ages or you are not sure what the other parents opinion is on the situation. I brought my daughter to the wading pool for the first time and we did not bring any toys along with us. One mother was there with her two toddlers boy about 18months and girl maybe 3. The whole pool was literally filled with between 20-40 toys; from barbies, matchbox cars, water wings, goggles, toy trucks, balls. ect. Of course my daughter couldn't help herself to want to play too, but every time she picked something up the little girl said "this is mine" or "this is my brothers". I wasn't really sure how involved I should get. I've actually noticed that alot of kids take authority better from an adult besides thier primary caregiver and I often want to help the other parent by telling the child they aren't being nice but I don't want to run into one of those over protective mothers and hear "Don't you dare tell my child what to do"(or "talk to my child like that")