I personally know two people who have switched from FM (or FM-IM) to Peds. Neither was permitted to advance directly to PGY-2, because they didn't have enough experience in pediatrics. At my program, we do 9 inpatient months, including NICU, general wards, subspecialty wards, and nursery, plus a Peds ED rotation and a general outpatient month. The FM residents at our institution MAYBE rotate in nursery, general wards, and general outpatient peds (they usually end up doing two of the three). One of my friends did a NICU rotation as a FM intern, but that was unusual. There's no way you would have the fund of knowledge to lead peds interns if that's all the peds exposure you had as an intern, even if the ABP/ACGME would allow it.
One of the people I know was able to get 2 months of credit that she did as an intern at our program as an FM intern-and thus graduated 2 months early. The other also transferred within his own institution, but chose not to take the credit so as not to worsen the call burden for his co-residents.