Under the provisions in parts 121, 125, and 135 [of the Code Of Federal Regulations] no certificate holder (US registered airline) may prohibit a child from using an approved CRS (child restraint system) when the parent/guardian purchases a ticket for the child.

~FAA Advisory Circular 120-87A Use of Child Restraint Systems on Aircraft page 7, Section 10 e, issued December 1, 2006