Welcome to Flyertalk! You didn't state how familiar you are with F9 and whether you've flown them before so let us know if you have additional questions. F9 is now a ULCC (Ultra Low Cost Carrier) so if you haven't flown with them in the past year or so things will be radically different than when you flew before. Make sure you read through their website to get an idea of how much it will cost to check bags, bring a carry on, sit together, etc. Obviously traveling with a family you want to keep things affordable but you don't want it to turn into a disaster.
If you have the Frontier credit card you can get a 5,000 point discount on award tickets with a paid ticket so that would certainly help your miles go farther in this situation. If you want the most bang for your buck I probably wouldn't worry about Choice awards. They can be a good value but you're spending twice as many points so it likely isn't going to be worth it.
In regards to seating please be aware that unless you pay there's no guarantee that you'll be able to sit together, however there is an interesting thread in the Early Returns forum here that discusses the algorithm that F9 uses for their seating assignments based on check-in time. However, since you're traveling with a lap child that won't be particularly relevant since you'll need to check in at the airport anyways. Seating assignments together are $3 per person per segment, so with a family of 5 that would be a total of $60 r/t.
Our family also has an upcoming flight to use of the rest of my points from SFO-MCO this spring and we've decided we're not going to prepay for seats since from what we've heard they'll at least try to keep one parent with each young child. One other thing to mention is that if you're only going from SLC to LAX or SNA it might be advantageous to just book Southwest (WN) and get a direct flight, free checked bags. By the time you pay for seat assignments, checked bags, and take into account the extra 3-4 hours from flying to DEN first, it might be worth it to just pay a little extra to fly direct.
Let us know if you have any more specific questions and we'll try to help you out.