@pleonasm - I find that putting a lot of ointment on makes it less likely that they will scratch, since it makes the scab feel more like skin and less like something that should be removed from skin.
I would also make sure that you or they are washing their hair the right amount. Kids don't have the same kind of oil production that adults do, so they shouldn't have to wash as often. Washing their hair too much might strip away the natural oils and harm their skin, leaving them more prone to sores.
If it sticks around, another possibility is that they might have something like ringworm. We were always playing with local cats and kittens and coming home with ringworm and it was particularly icky when it was on our heads.