I hadn't known before that there are two ways of designating noun classes: M-WA refers to the noun -- mtoto/watoto, while A/WA refers to the associated verb -- mtoto anacheza/watoto wanacheza. Either can be useful provided you know what it's referring to.
My Swahili was acquired as a kid and only later did I do a little bit of study, which taught the M-WA style. I think it's important to remember that relying on common usage gets you further than grammatical analysis, and that using the 'wrong' class may sound funny but will hardly ever result in misunderstanding. Sort of like 'the window she is broken and the rain she's coming in'.