Adventures in Toddler Bilingualism
Superfluous plurals.

My Yiddish- and English-speaking* 3-year-old, while playing blocks with me:

Du bist a boyer! ("You're a builder!")
Ikh bin a boyer! ("I'm a builder!")
Mir zaynen beyde boyersn! ("We're both builders'n!", i.e. with the plural form boyers and an added plural suffix-n)

*She also sings a lot of songs in Mandarin, but I don't think she can generate a sentence.