Stage 1 -- all they think about is whether they'll get punished. So your return only if you'll get in trouble for keeping it.
Stage 2 -- is governed by the Golden Rule. A kid at this stage might give the money back because that is what is fair and what they would want someone else to do to them.
Stage 3 -- they are most concerned with what their in group thinks. So they'll give it back if their in group would want them to.
Stage 4 -- you'd give it back because that's what the rules say to do.
Stage 5 -- you'd give it back because that's what society would want you to do.
Stage 6 -- you'd have to decide what's just in the given case -- what would be most likely to conform to some universal truth. So you'd probably give it back unless there were some reason that keeping it was more in line with universal truths.