We ran into a situation when we flew Southwest. It was myself, my 21 month old and my husband. We had paid for 3 tickets so we would all have our own seat and EBCI. We flew out early morning on the way down and had A16-18. We boarded first, got the very first row, installed our son's carseat, sat down and had the RUDEST people ask if we paid for my son's seat. I just said yes and they kept moving. The flight attendant also asked us. We showed her that he had a PAID seat and EBCI.
On the flight home, we were A10-13 or something in that range. We once again got into line and had a row close to the front of the plane. It was a CROWDED flight, PACKED. To the point where they got on to ask if anyone was willing to take a later flight for compensation. I had TWO flight attendants ask to see the ticket for my 21 month old. I had to show it TWICE. I was annoyed until I realized that the reason the flight was FULL was because a bunch of parents seated their children in seats that were NOT paid for. They were going row to row asking parents to see tickets. The family behind us were LIVID because they wanted a seat for their child but didn't PAY for a seat. They finally got off the plane.
I might be the only person in this camp but I strongly believe that airlines should require EVERY SINGLE PERSON, adult or child, to have a seat. It is ridiculous to sit a baby on your lap. If the plane hits turbulence, your child is in danger and then your child is a danger to others. Totally

but it annoys me.
With that said, I pay for EBCI. If I saw someone saving, I would ask them if everyone in their party had EBCI. If not, I would ask them to move. You get what you pay for PERIOD. If you didn't pay for everyone, MOVE TO THE BACK and save a row.