I've only gotten through about a third of the series, but I get the sense they never get back, at least not in the official show. So here's an idea:
During the extent of the show, the kids were there due to the manipulations of Dungeon Master (who, by the way, seems awfully "iffy" in terms of morality). Once they found their way back (outside of the show's plot, I guess after the final episode sometime), the time manipulation by Dungeon Master ended, and they were free to come and go as they pleased but time progressed equally on both sides. If the kids want to spend a week in DnD-world (I guess Forgotten Realms?), they have to come up with a valid excuse to be gone for that long. It would get easier as they got older, but then harder as they progressed their lives in the real world. After all, what parent would dip out on kids for two weeks while they chopped up dragons or whatever?
Presto, meanwhile, stays behind in the realms, but his life in the real world was nothing special. He had a real issue with confidence, but a lot of that is due to his unfortunate upbringing as a ward of the state. He's been hopping between foster homes for years, and while there might be an investigation due to his disappearance, no one would be surprised about his being a runaway.
Thoughts?