This is the 10770th most frequent Polish word.
dwulatkiem
A two-year-old child.
Here, 'dwulatkiem' is used as a noun referring to a child who is two years old.
Mój syn jest teraz dwulatkiem.
My son is now a two-year-old.
In this sentence, 'dwulatkiem' is used in the instrumental case, following the preposition 'nad,' which governs this case.
Widzę, że dobrze sobie radzisz z opieką nad dwulatkiem.
I see that you are doing well taking care of a two-year-old.
Here, 'dwulatek' is used in the nominative case to refer generally to a child who is two years old.