When I hovered under bad, it said "-baya". In that case, why wouldn't "mtotobaya" be considered correct?
It isn't indicating that -baya is a suffix. It is the root of the adjective, and it takes a prefix in accordance to the noun class. In this case, mtoto is a 1/2 (M/Wa) noun, so it takes the prefix m- (wa- in the plural).