the word אוהבת can mean both "love" and "like" so I'm not sure which one they want so I got it wrong :(
Why it can not be «The mom likes some milk»? In the tips and notes it is explained that when you do not have the definite article, the indefinite is understood.
Well, this sentence simply states that the mom likes milk, in general. She doesn't like the milk (החלב), she simply likes milk. If you wanted to say the mom likes some milk, there are a couple options. If you wanted to say that she likes some, but not all milk, you would have to complicate the sentence, and say האמא אוהבת סוגי חלב מסוימים - the mom likes certain kinds of milk. If you wanted to say that she likes a bit of milk, you would say either האמא אוהבת מעט חלב or האמא אוהבת קצת חלב. The words מעט and קצת have subtle differences that I don't think are worth going into detail at this stage.