You can layover, but the non-prohibited campsites are all below the Grand Gulch campsites. You'll still have to do long(er) days through the majority of the lower canyon, then short days in the last few miles. I recall that there's not that many quality non-reserved sites in the last 10 miles or so. Seems that there were a few options upstream of these sites, but I believe most of those aren't very kid-friendly (very rocky with drop-offs into the river.
This map doesn't show additional sites, but it does show the restricted ones.
One thing about campsites in this stretch that needs a reminder: all of them are basically at the mouths of side canyons that can and do flash flood frequently. Be aware of this when you tie up and set up tents and kitchens.
Even if you are mindful of the side canyon flashing, also know that the San Juan can jump up huge with rainstorms miles away. We camped at Oljato one night, heard thunder but no rain, and I went back to sleep. Grand Gulch had flashed upstream of us (taking out a party's whole kitchen) and the river levels where we were rose 3 feet. Luckily, a couple of late-nighters in our crew just managed to save our rafts and a bunch of gear from getting swept downstream. We did lose a cooler, though.
Have fun- we're taking our 2-1/2 year old down the Bluff-Mex. Hat section in June~