"Seu" is not really easier, because it can mean "Your/His/Her/Their" all at once - without context, you don't know which one is the correct, and that's precisely why "dele" (his), "dela" (her), "deles"/"delas" (their) exist - to provide you with ways to unlock the tangle of meanings created by the regular 3rd person possessive.
Why is "bebe" = "is drinking"?
My (wrong) answer: "Their monkeys drink milk".
Shown corrected translation: "Their monkey is drinking milk".
I am still that weak on grammar, but maybe someone could link the related skill or forum discussion so I could review that specifically (or point to Memrise course/level for more practice).
"beber": that is sometimes the "to drink" basic forum, right?
O macaco (the monkey, masculine) delas (de+elas, of them, feminine, e.g., as mulheres, as pessoas) bebe leite. Portuguese doesn't have an equivalent to "their" in English; "O seu macaco bebe leite" could be translated as your monkey, his monkey, her monkey, their monkey. In order to be specific, you use the construction "the ____ of him, of her, of them". So, "o macaco dele" is his monkey. "O macaco dela" is her monkey. "O macaco deles" is their monkey, either a group of guys or a mixed group, and "O macaco delas" is their monkey, they being a group of solely girls.