Well, kind of not so fair to others. Here's the thing. Your dh would have to save 6 seats..and that is not going to make him so very popular. That's two complete rows!!!
I would try to use the one car seat as already mentioned. If your 2 y/o is normal sized, you should be fine. Makes less to juggle at the airport.
As already noted, you most likely will have no issues. You will all be able to board in between the A and B groups. As noted, if you head to the rear of the plane, you should have no issue getting two entire rows together. I would recommend taking two consecutive rows, not next to each other. Take the last row, put the kids in that row, and then you you and the little ones sit in front of them. Put dad across the aisle, last row from the older kids. That way, if the older kids get nudgy, they will be nudging your seat, not a strangers.
I also like to pay for EBCI for the return flight. That way, you don't have to worry about being close to a computer to check in. But, you will most likely be able to board the same way you did flying down....family boarding.