I don't see how it could be a pay cut. Actually, I can believe it happens, but it doesn't make any sense. A Peds EM doc will be board certified in both adult and peds EM. It seems if there was a big pay cut for Peds EM docs then they would just stay with regular EM jobs. A pay cut for peds EM docs is the equivalent of saying the average cardiologist/nephrologist/rheumatologist is payed less than the average internist who only practices internal medicine.
I guess one plausible possibility is that Peds EM docs are paid less than the average EM docs in the US because a higher percentage of dedicated Peds EDs are in academic environments where overall compensation is lower.
Another possibility is that Peds EM positions are in higher demand by physicians so hospitals can get by with paying them less.