This sentence could never mean : Babies drink milk, right? I sometimes get confused by Persian where generalisations can also be made using the singular...
It most certainly cannot mean that. you would have to say "bebekler..." :) You do have to mark subjects for plurality in Turkish.
In turkish if you just append '-ler' or '-lar' to any noun it will become plural, so simple
Because it is a general direct object. "Bebek süt içer" is a general statement (The baby drinks milk). "Bebek sütü içer" is about specific milk (The baby drinks the milk).
In turkish if you append 'a,i,o,u' to any noun, that will mean kind of 'the' in english. Basically a noun is very general, kalem=pen, milk=süt,
The pen = kalemi, the milk = sütu