Various types of tape can be put all over the opening around their butt, which can be removed if soiled, or just wiped with a wet wipe. When I did peds ortho, we would wrap moleskin over the sharp plaster edges and then wrap plastic-coated tape over that.
As for the constipation itself, try to minimize opioids obviously, but I'd start with a stool softener. It won't make them go, but it will help them out when they do go. Then I might try a bisacodyl suppository, since it shouldn't give them diarrhea, it should just help things move along. After that, Miralax is probably a good idea, and it shouldn't cause blowouts unless they're getting a lot of it.
An enema would probably be my last choice, since that seems almost certain to cause a blowout in a kid.
