Why can they not cross the sideline?
3-7-3. "During the same dead ball interval... no player shall withdraw and re-enter as a substitute unless a penalty is accepted, a dead ball foul occurs, there is a charged time out or the period ends."
In practice: as preventive officiating, I tell the kids who are already on the field "don't cross the sideline", and tell the coaches that anybody can come out onto the field to join the conference. That way nobody's left the field, and nobody can argue that anyone's withdrawn and returned in the same interval. Am I going to be a hard case about it if the kid takes one step onto the sideline to get a water bottle from one of the attendants? No, but if I keep the players on the field on my sideline, and my partner keeps the players on the field on his sideline, then we don't have to worry about it becoming an issue with either side.
EDIT: I know that an issue with this would be looking for the figurative fly poop on the pepper. But - to mangle the metaphor - if I can say something to prevent the fly from getting anywhere near the pepper in te first place, I don't have to worry about the poop anyway.