When I was a kid, I always used to drink milk.

Literal

I [topic-は] child [possessive-の] time, always milk [object-を] was-drinking [insistence-よ].

~ていた expresses a habitual past action. いつも ('always') reinforces the habitual nature. 牛乳 (ぎゅうにゅう, 'cow's milk'). Sentence-final よ adds assertion.