That is normal behavior. They go inside because they don't feel comfortable looking outside. Sooner or later they will go voluntarily. But the way I "forced" mine is to have a door that locks them out and they can't get in. I do that with my 3 weeks old babies for 30 minutes to 1 hour for 3 days, then I put them back to their parents. That is their first training here at my loft. They seemed to have associated food call as well during those times. Once they get weaned, they seemed to have curiosity and they will venture out to see the outside with settling cage of course. Just give your birds time for now and train them food whistle first. Trap training comes later, then outside observation with settling cage(the one you are doing now) and finally unforced release. I think you can all do these in 4 weeks. The only thing I am worried is that by 4 weeks, they might be wing strong and if they took off suddenly, you can lose them. But that is the negative side.
So the plan of attack:
First week: Let the birds feel comfortable first and food whistle train.
Second week: Trap training. If they are smart birds, they can learn to trap in 3 days! You may wish to expose them to the outside now, too, with settling cage. Let them get inside the loft in respond to food whistle or call.
Third week: More surroundings exposure with settling cage. If they don't go outside voluntarily, then you may be force to catch them and put them there. Let them enter the loft with food whistle or call.
Fourth week: Take out the settling cage and let the exit door open so that they will come out voluntarily. Don't force. Let them come out and in voluntarily and be there. If you are lucky and they don't take off suddenly, then they will try to learn their surroundings more either by flying to the loft roof, etc. Sudden noise, hawks, cats can scare the hell out of them and they can get lost.