I IMHO opinion using miles for a first class for a 2 yr old is a waste. A 2yr does not require the space in first or the amenities. So how can the adults enjoy the amenities of first while getting the best value for a child that does not need those amenities?

Maybe a couple of options

1)Get one seat in first and the other two seats in Y. One adult enjoys flying in first class while the other adult and child sit in Y. Then on the return trip the adults switch. As one poster mentioned it is hard to get your 'first class' when have a child to focus on. This allows each adult to enjoy first class and saves some miles

2) get E+ for all the pax. Not first class but at least you get the extra space.

3) I see this all the time in the middle east, parents fly first class and the maid flies econ with the kids. Well we don't have a maid but we have thought about paying for a family member to fly with us. The family member can also act as a baby sitter while parents enjoy some alone time during the vacation. Of choose this comes at extra costs since you probably will pay an extra ticket and hotel room on vacation. But alone time for Mum and dad on vacation is priceless.

we have done the first option on 14 hour Trans Atlantic flights from middle east with my wife and I switching our seats in mid flight. This allows each of us to get some undisturbed sleep on lay flat seats in business class.