Last year, the first through the 7th, it was in the high 80's and low 90's. The 7th -10 it was in the high 70's. September, we usually start to have our "heat waves". I dont know exactly what the water temp will be but if the weather is still hot, it should be warm too.
Your kids will be able to swim and the lifeguards will still be there. Plant yourselves next to a life guard tower for extra security.
Labor Day weekend will be a zoo. If the weather is hot, like the 90's, all the people inland will be coming to the beach as well as the locals. They will make a full day of it and all the fire pits will be gone very early. After Labor Day, it will be a whole lot slower. because school is in session.
Weekdays are always better at the beach especially if you want to get a fire pit. Fridays and Saturdays, the local crowds like to do bonfires but you dont have to get there by 2 or 3 to acquire one like you will have to do on other weekends. (Labor Day weekend you probably need to get one by 11 am or earlier depending on the weather.
Main Street is chalked full of Mom and Pop dining. You can take your pick. Dukes on the sand also serves seafood and terrific calamari..not Mom and Pop..but very nice. Your kids would probably like to eat at Rubys at the end of the pier over the water.
I have some photos in my profile of Rubys, Dukes and the beach.