"That child is drinking milk."
Translation:Tamto dziecko pije mleko.
Yes. Or more specifically, 'tamten' is for masculine singular and 'tamto' is for neuter singular. (and 'tamta' for feminine singular)
Mleko (like most nouns ending in -o and -e) is a neuter noun. For neuter nouns, the nominative and accusative forms are the same.