I did the same thing you're describing (only my coop is permanent, not a tractor). They were about 9 weeks when they went from the house to the coop and attached run - out during the day, closed in at night. After about 3 or so weeks I started to let them free range during the day. I have no problem getting them in the coop at night, that is when I refill their feeder and initially I got them in with treats (also I spent a lot of time with them when they were chicks so they are pretty tame). They tend to go back near the coop when they are resting, and then venture out and forage periodically.
I have llamas in the same pasture so I don't worry too much about daytime predators other than hawks. I agree about supervising your birds if you don't have a guard animal or a predator proof pasture/yard.
Good luck!
Melissa