I don't know that I would ask people to bring sides.
I'm not sure why . . . maybe because it's your first party with these people? Maybe that's a bit weird. But that's just me.
Of course, if anybody offers to bring something, there's absolutely nothing wrong with taking them up on their offer.
I don't think your menu needs to be extensive . . . plenty of hotdogs, chips/dips, beans, probably a potato salad and/or a coleslaw (make sure you have a way to chill these all afternoon) or a fresh-veggie tray . . . add a sliced watermelon with the ice cream and you're good to go.
Great idea on hiring the babysitter for the pool area!
HOWEVER! Does she have any life guarding experience/training? Make sure the parents know that she is not solely responsible for their kids . . . Let the parents know that if their child is in the water, they need to be in attendance as well. (Water safety is extremely scary with little ones.*)
Make sure your babysitter is well supplied with fresh drinking water and that she gets a short break every hour or so.
Also, you might want to have a supply of sunscreen on hand.
*I, too, had to jump into a pool fully clothed while my DDs were being "watched" one-on-one by two teens. The one teen watching DD#2 (3yrs) swam away to grab a ball, leaving DD#2 sitting on the in-water steps. When I looked up DD#2 had her head under water and her arms out, several feet away from the steps. Yes. Probably the scariest moment in my life.
And, yes. I posted that to scare you. It only takes a moment.