Iv'e been to quite a few beaches on the cape with and without the kids and here are my picks.
1: Old silver beach in Falmouth. Good sand low waves (no rip tide) and plenty of beach. I would plan to go kind of early on a weekend due to the popularity of this beach. If your planning to stay a while I highly recommend the Seacrest resort. It's right on old silver beach but it has it's own privet section with a bar just off the beach. At night is when I brake out the fishing gear
the lights from the restaurant attract Bait fish and scup in to the swim area and live lining scup off the jetty that runs along the out flow of the near by salt pond is a great way to get big cows. I have a few days reserved for my family this summer.
2:Nauset beach in East Orleans is a great beach. a bit cold this time of the year and Because it's facing the Atlantic it's prone to big surf and some times rip tides. the public swim area has life guards, rest rooms and a snack bar near by. there's plenty of reasonably priced parking and usually more than enough beach for every one. When we go to naust we walk to the right just beyond the life guard tower were it's OK to fish and I put out a sand spike and Dead stick a sea worm.
I can't think of any beaches on the bay side that aren't great But Brewster and Denis are my favorit. Be aware of the tide in Brewster However, because at low tide you will have to walk about a mile to get to the deep end But the walk is one of the reason I like Brewster.
I hope this helps. I could go on all day but you get the jist.Plymouth has great beaches and is a bit closer if your just looking for a good day trip. Check out Horseneck beach.