These days everyone seems to moving to some sort of productivity based model, so pediatric dermatologists don't really make less money per unit of work compared to anyone else.
The main reason why pediatric dermatologists get paid less is that they do fewer procedures (biopsies, ED&C, LN2). They do fewer procedures because they mix of diseases they see don't require (as many of) them. It probably takes longer to see the average child vs the average adult derm patient, so that might factor in a bit. But it's mostly just fewer procedures.