Ethics of giving students gifts?
Many institutions have a dollar limit on what is considered a small gift that does not present a conflict of interest or other unethical circumstance.  $10/person is probably well below that limit but you could probably check with admin.

However, that does not prevent the students of the second group perceiving unequal incentives for the different projects.  It could be de-motivating for them to see the other group being materially encouraged.  It would be ideal to somehow equalize the incentivizing of both projects.