I was in the same situation -- I needed 5 tickets, so I called the number on the web site to see if there was any way that I could book all 5 at the same time and not 4 and then 1, for the same reasons you mentioned. I explained to the agent at Spirit what I wanted to do, and she told me that I would have to do 4 and then 1 in two separate transactions. However, she asked me what dates I needed and she was able to check and tell me how many seats were available for that flight. (There were quite a few for the flight I wanted.) I then booked thru the web site. T hey give you a confirmation number afterwards thru email, so you'll know you're okay.
Also, I think they have a new feature where you can choose your seats. (THis is for tickets booked online after April 12th -- I booked before April 12th, so this feature was not available when I booked.) I would check with the agent about choosing your seats. If, for whatever reason, you are not able to choose your seat ahead of time, I know that seats are assigned on a first-come, first-served basis, so get to airport in plenty of time and you should be okay. We have 3 small kids and we have never been split up from our kids. We have been seated in the same row, 2 on one side and 3 across the aisle.
Hope this helps!
