Is being a little the same as being submissive?

No, though they overlap often enough that people conflate them. Submission is about who holds authority. Little space is a headspace: playfulness, comfort, and handing over day-to-day responsibility rather than power.

There are littles who are not submissive anywhere else in their lives or their relationships, and who would bristle at being ordered around outside that headspace. There are submissives who have no interest in little space whatsoever.

The other thing worth saying plainly: none of this involves minors, in any form. It is adults choosing a headspace with other adults, and every community worth joining treats that as a bright line.