This is a set phrase that people use when offering drinks to visitors to their home. It sounds polite and friendly, and adults use this phrase with all kinds of guests, from friends to repair people to colleagues.
You can also ask:
Can I get you something to eat?
Can I get you a drink?
The difference between "something to drink" and "a drink" is that "a drink" usually means alcohol, while "something to drink" can mean water, juice, coffee, etc.