Which is great, in an urgent situation caused by the airline (such as a delayed connecting flight). But when an airline offers an option that virtually guarantees passengers can sit with whomever they wish - Southwest's EBCI, AirTran's or Spirit's seat purchase option... - and the parent instead expects the airline to fix any seating problems, that's just inconsiderate.
I agree! I just flew Spirit May 5th and 12th. Knowing their seating policies and that I don't do well in middle seats or seats in the back of the plane, I purchased one of their 'Big Front Seats' in the front of the plane. Hoping that the price would go down closer to departure I did not purchase it until I checked in 24 hours before my flight, but did keep a close eye on them to make sure they were not selling out. When I got to the airport I sat close to the desk, as I always do. The gate agents didn't show up until just before the flight next to us was getting ready to board, but I heard several families having a fit among themselves because they were not seated together.
Our gate agents finally showed up but started boarding the gate that was right next to ours first. As soon as they walked up, it was funny to see the number of people from our gate running over to mob them. I heard them telling them they had to get that plane boarded, then they would deal with any problems or complaints from our flight. After that plane was boarded the same two gate agents came over to our gate and again were mobbed by people demanding that their seats be switched around. You could see the one gate agent take a really deep breath, then rather loudly made the announcement that Spirit makes it perfectly clear on their website that if you want to sit with your party then you need to purchase your seats. He then looked at the group standing in front of the desk and said, "If you did not purchase your seats, then you were assigned whatever seats were left over and those are your assigned seats. If you can get someone to move for you once you get onboard, then that's up to you, but we will not force anyone to move for you. They paid for their seats and have a right to keep them. You really should have done the same. Now if anyone has a problem with anything unrelated to that matter, step forward. Everyone else please sit down. There will be no reassigning of seats by the staff today" He was fairly young, so I was surprised at just how forceful and unbending he sounded, but impressed at the same time.

What surprised me were the number of adults that were complaining and asking for them to move people, so that they could sit with other adults.

Up until Air Tran left Dallas this past Nov, I used to fly them several times a year and every single flight there was some family raising heck with the gate agents that they had to move someone around so they could sit with their kids. Sometimes the GA or FA would ask and sometimes they wouldn't, but they never made anyone move. It's not fair to those who pay for their seats to have someone who didn't come in and ask for them to move.