trip from SLC to Vegas), you can't select your seat unless you pay extra.

On the way to Vegas, I checked in for my flight right at 24 hours before departure. The computer stuck me in the very last row of the plane next to the restroom. Fortunately the gate agent moved my seat to the front row upon boarding. But I learned that the sooner you check in the worse your free assigned seat is.

On the way back home, I went to check in the day before the flight and was offered better seats all around the plane if I paid some money. Instead, I didn't complete the checkin process. As the departure time approached, more and more people checked into the flight and the back rows disappeared as available options one by one. I waited until 90 minutes before the flight to check in, while I was already at the airport. By that time, almost every seat had filled up. I was automatically assigned row 2 for free because there were only a few unassigned seats left. So it looks like Frontier punishes you for checking in early if you don't pay extra for an assigned seat.

Having read the OP, I probably won't fly frontier again (especially if I have young children with me). But if I do, I'm waiting until the last minute to check in so I can get better seats.