"Registering" is just getting a beeper, the kids get arm bands, and you fill out a form concerning allergies, who can pick them up, etc.
After that, you can take them to the club or lab essentially any time except when they are moving from one location to another. Then, you wait about 10 minutes till they are at the new location. There are occasional notes in the navigator saying things like "no check in or out during the movie" as a safety to the kids as they will be in a darkened theatre and staff would have difficulty identifying them and you.
Very occasionally, you may take a child and find that the club or lab is already at capacity (kid to adult ratio is full.) In that case, they will ask you to come back in an hour or so. Of course, you can always wait till a kid checks out. This doesn't happen often, but is a possibility. Since kids are constantly checking in and out, it doesn't stay "full" for long.