1. Frontier does not fly "over the Pacific" (unless you consider landing to the East on approach to SFO "over the Pacific") so OP is perhaps complaining about the wrong air carrier.
2. There is no US law requiring that families be seated together.
3. Even countries which require an acommodation only do so for one adult not the family. Thus, in your case, if you want to be assured of a 2 YOA seated next to a parent, you would pay $20 or $40 (not $200 as you state).

Rather than posting vague terms such as "over the Pacific" please post your entire exact routing.