We are going to the States with DCs 10 and 8. The 10 yo is nowhere near the minimum height to not have a child car seat in the UK. I rang the airline to ask if I could check my own in. The woman I spoke to was both clueless and snotty. 
Im not just concerned about the legal side of things. I would like to keep my kids safe! We are visiting people who do not have young kids. Should we buy seats there and hopefully internal flights will be more accommodating? Any of the MN U.S. contingent can advise?