"Hi ydy'r mwya plentynnaidd."

Translation: She is the most childish.

[rhywun]

why does it consider including the final f on mwyaf as a typo?

[ibisc]

A mistake, sorry. Unfortunately, this sentence can no longer be edited as it has been removed from the course database and is awaiting final removal by Duolingo. It has an error in it and should have read:

• Hi ydy'r fwya plentynnaidd - She is the most childish

The superlative mwyaf has to change to fwyaf in order to agree with the gender of hi.