You're right, you can bring two car seats on board for free, but be aware you won't be able to have them in the same row since car seats always need to be in the window seat. Also, on Frontier you can't bring the car seat on board for the lap child unless you purchased a ticket for them. You'll need to check or gate check the infant car seat.

Frontier won't assign you seats in advance together without paying the seat selection fee but you should be able to get at least one of you seated with the toddler and all three of you if there are three seats together when you check in at the airport.

If you want my recommendation for how to do luggagefor cheap, I would buy a car seat carrier from amazon.com or ebay and pack any extra room full of clothes for "padding." I have done this when flying Allegiant, Spirit, and Frontier, and never had a problem. Put any other fragile items in your backpack as a free "personal item" and enjoy your flight, well as much as you can enjoy a red-eye flight on Frontier with two kids.