We had the same kind of dosing problems. When Hailey started hers at 2 months, I can’t remember her weight, they said that the highest dose they could give was 7mg/day. … they said something about 1.4mg/kg twice daily as a MAX dose or something like that. I didn’t know about marci-kids, so I didn’t know that we had room to move. I would monitor her symptoms. If you don’t see any improvement and she needs more, then I’d try to get a doc who will increase it, as there’s definitely room to move.
Good luck. Hope it helps her.