The demand is high for peds rads for the reasons that mammo is in high demand. You have high liability and low reimbursement => nobody in their right mind went into those fields leading to a severe shortage. Even though those people do not bring in the RVU's, they perform the 'dangerous work' so the high RVU people (neuro, msk, IRs) don't have to. In turn those people can crank out the RVUs for the radiology group, instead of tiptoeing around in the minefields. Its a win-win situation for everyone. Most hospital contracts want groups to cover peds and mammo, so somebodies got to do it. Its comparative advantage. Sure plain films don't bring in RVUs, but the efficient old-timer plain film reader can crank them out, so that the MRI reading rads don't waste anytime on non-MRI studies.
Bottom line, do what you like. If you like peds, do it. But don't do it just because it'll get you a job. By the time you start fellowship, the pendulum might swing the other way.
MRI based fellowships were en vogue 5-10 years ago, because not many old timer rads had experience in it, and MRIs were a cash cow for the group. MRI trained rads were a hot commodity. Consequently everyone did this fellowship. Now adays you should get some decent MRI experience in residency. What we are seeing now is an oversupply of MRI (neuro or msk) manpower.
Most jobs out there are still for 'general' Radiologists. The highest subspecialty needs seem to be in Mammo, Peds and then IR. Again by the time you MSIVs finish residency this could (and probably will) completely change.