You're reaching now. By July 15, 2017 the DOT should have a final plan to require that children 13 or younger be allowed adjacent seating at no extra cost (i.e. no premium window/aisle charge) with someone over 13. It's Public Law 114-190, which was HR 636 - passed July 15, 2016.
https://www.congress.gov/114/plaws/publ190/PLAW-114publ190.pdf
SEC. 2309. FAMILY SEATING.
(a) IN GENERAL.—Not later than 1 year after the date of enactment of this Act, the Secretary of Transportation shall review and, if appropriate, establish a policy directing all air carriers providing scheduled passenger interstate or intrastate air transportation to establish policies that enable a child, who is age 13 or under on the date an applicable flight is scheduled to occur, to be seated in a seat adjacent to the seat of an accompanying family member over the age of 13, to the maximum extent practicable and at no additional cost, except when assignment to an adjacent seat would require an upgrade to another cabin class or a seat with extra legroom or seat pitch for which additional payment is normally required.
(b) EFFECT ON AIRLINE BOARDING AND SEATING POLICIES.— When considering any new policy under this section, the Secretary shall consider the traditional seating and boarding policies of air carriers providing scheduled passenger interstate or intrastate air transportation and whether those policies generally allow families to sit together.
(c) STATUTORY CONSTRUCTION.—Notwithstanding the requirement in subsection (a), nothing in this section may be construed to allow the Secretary to impose a significant change in the overall seating or boarding policy of an air carrier providing scheduled passenger interstate or intrastate air transportation that has an open or flexible seating policy in place that generally allows adjacent family seating as described in subsection (a).
Most airlines do in fact hold back some seats that can't be selected until either check-in time or only by an airline employee. I've been through that on a regional jet where there was no option for premium seats because it was all aisle or window. All we could select were scattered. When I got to the gate, one seat next to one of ours magically became available. This is all planned for in order to accommodate families with children or any passenger who might require an attendant. Heck - if it's a disabled passenger requiring an attendant, the attendant legally flies for free.