The link provided above applies to people flying from Canada to / from (or over) the US.
Children under 16 do not need a passport to fly domestically within the US (or to cross the border by car). They just need some proof or age and residency (e.g. Canadian birth certificate or health card).
(we flew on from St-Pete to Ogdensburg last Sunday and although we do have passports for our kids, we were told to put them away as they only needed the adult's)