Perpetual motion machines

The kittens are ~6-1/2 weeks old and, after a minor slowdown during the
hot weather, they're back to full speed exploring this morning. Two of
them have learned to jump up (as opposed to hooking in and pulling
themselves up) well enough to get into the bathtub. (Time to cat-proof
vertically.)

Anyway, anyone have any thoughts about what, if any, long term negative
impact caging kittens at this stage has? I do understand that it's
necessary; animal shelters don't have the room or personnel to do more.
The kittens also are still doing a lot as a group; one finds/does
something new, the others come check it out. What can an adopter of a
single kitten do to make up for what it has missed ... or doesn't it
really matter in the long run?