Was in and around Plymouth,NH today and earlier this week, the looked pretty low. I'm an experienced pemi tuber myself from my days at Plymouth State and the section above Livermore Falls looked undoable around the Blair Covered Bridge (wouldn't take kids there anyways), below Livermore Falls is usually doable on a tube in low water all the way to Plymouth.
This probably isn't the section you're thinking of doing but if it's low at Plymouth it's real low upstream.