"My child is slim"
Translation:Mtoto wangu ni mwembamba
-embamba is slim/slender/skinny/thin/narrow etc.
You have to add a prefix to agree with the noun class. In this case, mtoto refers to a singular animate being (person or animal), so the prefix is m(w)-. The (w) is included when the next letter is a vowel.
There's a table of all the prefixes for -embamba here https://en.wiktionary.org/wiki/embamba
correction: mtoto wangu - watoto wangu.
It's probably a word origin thing, but there are differences when it comes to prefixes used with kin names. Mama, baba, dada, kaka etc are all 'yangu'; mtoto, mzazi, mume are 'wangu'. I can't remember which lesson had notes on this, but I'm sure I've come across them before