I love Spirit, but you need to know going in what the extra fees are or they will get ya. A personal item that fits under the seat in front of you and is no larger than 16"x14"x12" is free. Anything bigger than that would be considered a carry on and would have to go in the overhead bin and you would be charged a fee for it. The fee for carryons is $35.00 ($25.00 if you are a member of their $9.00 Club). I've never seen or heard of them weighing carryons. I did see them measuring a lady's personal item though, so if you plan on using a personal item under the seat, make sure it is no bigger than the 16"x14"x12" that they allow for underseat items. Checked bags are $30.00 ($20.00 if a member of their $9.00 Club). Those fees are each one way of course. You'd have to check online to see if they have any fares available for your dates that might be cheaper if you are a member of their $9.00 Club and then add the price savings up to see if joining their $9.00 Club is worth it. It costs $59.95/year to join their $9.00 Club. On the flights I've taken on them they have never had any of the cheaper fares available. WARNING: They are the one airline that has a lower weight limit for checked bags than most airlines and if you don't know about it you will get called for overweight bags. Their limit for a checked bag is 40lbs, not 50lbs as seems to be industry standard.
Since Spirit is known for it's many fees I don't know if they'd sit you with your 3 y/o or not, but would definitely not have a problem with your 6 y/o sitting alone. Heck, I've seen Air Tran seat 2 and 3 y/o's apart from their parents. usually someone volunteers to move so they can sit together, but Air Tran is not going to make someone who paid for their seat move, for someone who didn't, and I don't think Spirit would either, so I'd pay for seats just to be sure. Heck, everytime I've flown Spirit I've paid for my seat, even if I'm solo, because aisle seats close to the front are important to me. Alot of people feel the same way and would not be willing to trade for some middle seat, just so some passenger who chose not to pay for seats could sit together. As far as seat fees: the 'Big Front Seats' (those most airlines would consider first class) are around $50.00 each way. The seats behind them, but are still closer to the front and also the exit row seats are $25.00, seats just behind those $25.00 seats are $18.00, and seats farther in the back (like from row 14 back) are $14.00.
