Why is "wangu" plural? "My" is singular, so shouldn't it be mngu instead? Or is it just a pronoun that looks like it has a plural prefix?
I don't quite understand your question, watoto is plural, my is an adjective, in English we don't have different forms to agree with the noun it's describing, so my does for plural and singular. In Swahili the noun needs to be agreed with both by noun class and number ( singular or plural) so as watoto is plural the there's a plural prefix for 'angu' w, making it wangu.