I understand.
Lets work on the dual motors first (if OK with you). I think you should go with the Fet you originally mentioned. There may be something slightly better but not by much if at all. And I'll guess the cost is small compared to the motors and it will handle anything you throw at it.
So:
- To get the low Rds on and reasonably fast switching you must have to drive the gate with the full 3s lipo voltage (see the data sheet fig 1 and fig 2). You will need a pre-driver to quickly bring the gate from 0V to 8- 10 V. Perhaps a driver like the UCC27532 could help.
- The MosFet will require some sort of heat sink. Mostly for the repeated startups. My guess is that much of the energy to actually launch the nerf dart might come from the spinning inertia of the motor armature and launch wheels.
Next question, why dual motors and not one motor driving both the wheels. It would solve the problem of the motors rotating a slight different speeds when the nert projectile hits the wheels.