For starters you can only have 1 lap infant per row (on a 737 sized airplane). This has to do with oxygen mask availability.
They are not going to care who is sitting in what seat on the plane in your own group. So you will not have to switch seats on the reservations. Technically you can install a car seat in the middle seat as long as it does not block egress of the window passenger but on the plane that might be an FA call.
You will want to link the reservations so that if a rebooking happens they "should" keep the reservations together.
I am sorry to say it may end up causing drama either way. Someone is going to have to be re-seated if you have 2 lap infants all in a row. The FA might make the call the car seat has to be in a window seat.
Last edited by firespirit; Jul 7, 2019 at 12:38 pm
Reason: Correction on oxygen masks.