I don't completely understand what the Seedling Tails obtained in the final episode is: Cosmo rigenerated,her child или just a plant?

When Cosmo gets regenerated it is actually her child but it will grow in many years time.

i belive that cosmo is a plant and a child

I don't know for sure, but I think it would be her child. Cosmo is a Seedrian, growing as a plant. Therefor, I believe her child would grow for a long time as just a plant, but at some point be human, or, well, Seedrian. This is just a guess, though....