2 day theme park/water park

Na

I need some suggestions. My husband and I are taking a 2.5 yo and a 4 month old to a water/theme park. A set of grandparents will be there, so plenty of hands. But i want to make sure baby naps and stays cool when not in the water. We will have the double stroller with the car seat. I am overweight and havent been able to find a carrier with either kid that works. Any suggestions aee welcome.