Aloha e @komota15 ; this is confusing and should be clarified on Duolingo. "A toy" in the English sense would be "he mea paʻani" (you are correct). But in Hawaiian "kekahki" is typically translated to "another, other, one of the, some of the, etc..." in English because it more closely follows.
So really itʻs more like the following:
- he mea paʻani. A toy
- kekahi mea paʻani. A toy (out of a group that you and I know about and are referencing).
So essentially they are the same, but with an implied context for kekahi.
Hope this helps! (also I saw your other post on this before so, e kala mai, I left you two responses)