The front crossbar seats work really well. I regularly (4-5 times per week) go out on my bike with a DS1 on a trailgator thing, and DS2 on a front crossbar seat.
The seat I have is a Centric Safe Haven, which is an excellent model, it has straps, a headrest thing so if the kid falls asleep on the bike, they have somewhere to lean.
You really can't put a back seat with a trailgator, even if the back seat doesn't attach to the post (the model I have goes on your rack), the bar for the trailgator would have to go through it somehow.