To be honest BenjaminNYC - I don't think it's a good idea for an infant to be "lap child" on a flight (regardless of length). However, I am amazed at the fact that there's no requirement for a safety device (such as an FAA approved car seat) once the child is 2. Have you seen a 2 year old wearing one of those lap belts? They're beyond useless - my arms are more "supportive" - the child would slide thru the bottom of that lap belt in a fraction of a second - I have no doubt that they would be useless in an emergency... Hence why I would ensure that my child was in a booster/safety seat until they're older and more secure in the seat.

However, to the OP's question, just make sure that the child doesn't turn 2 during the journey (i.e. the return occurs prior to the 2nd birthday) - otherwise per the rules the child would require a child ticket for that leg.