Obviously if you are taking them to the park on Shabbos, you are taking them in play clothes, so if they get wet, they get wet. If it is so hot that they need to get wet what’s the problem, they will dry off. If you need to dry them off don’t take them to that park or let them know that if they go into the sprinklers you will take them home immediately and not go back next Shabbos.
The problem is not the park or the sprinklers the problem is your relationship with the kids and them not listening to you. You have to say what you mean and mean what you say. If you say NO then it has to mean NO and the kids have to believe that. If it means that YOU are stuck in the house with them as a consequence then you will all have to be stuck in the house until they get it.