If you go up from the swimming hole by the picnic areas a little, you'll find a spot or two with chest deep water or therabouts, and if you go down just a bit from Little Yosemite, there's this marvelous spot, maybe ten feet in diameter, that's over my head (I'm 5'6"), has a highish but small in volume waterfall pouring into it, and a rock near the water that's great for jumping off of. It might not be very safe if your son is particularly young, though. And there's no way you're going to be able to do laps in it, if that's your intent.

Just generally, though, if you're feeling particularly adventurous, hiking up the stream itself from the picnic area is great fun. There are some really awesome spots that there's no way you'd experience if you stuck to the trail. Old shoes are a bit of a neccessity, and a good sturdy stick might come in handy in some areas or help speed you up in others, but with them it's a real marvelous adventure.