I'd go for 3+1. That would let a couple sit on the other side. This would give you access to the aisle. It would also be easier to trade if it didn't work out.

The three year old might kick the seat in front. In that case, if it's really a problem, you can offer to do 2+2, rows in front and back. The 3 year old might like sitting at the window.

You can't do 2+2 on the aisles because of the car seats. Be sure to bring the car seat for the 3 year old on board if you need it at your destination. Don't check it as luggage and even gate-checking it isn't ideal. You'll find it much easier once it's on board. Since you have the luxury of flying with two adults, then it wont be a big deal to bring it.