We still fly with the boy on our laps - he's pushing 19 months now.

I purchase, and fly, first with him.

The flight attendants are great about this. Fellow passengers have been incredible on every flight, e.g., playing with him when he peeks over the seats, telling me that their kids cried all night on a red eye to Europe, etc. The extra room works for him because he's not able to kick the seat back in front of him, and he has a bit of room to roam on the floor. And there's no issue with moving between cabins when he needs to get up and move around.

I'm not sure that I would choose three first class seats over three coach seats when travelling with a toddler though. In coach you have the entire row and I think it might be a bit more convenient. In domestic first you'll generally have two seats together, and either one of the adults gets really lucky the entire flight or you'll switch off during the flight.

I would think through the flight dynamic on that basis.