"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