Do you like milk? You'd better. Milk is good for your bones.

-Drink your milk, kids.
-I don't want milk. Milk's for babies.
-Yeah. Babies.
-Oh, yeah? Well, I happen to know that milk helps build strong bones. So drink up.
-Well, Mr. Miller told me he never drinks milk. Look at him!
-Yeah.

-Hi, kids! Ohh…that’s not good.

Got milk?

I HAPPEN TO KNOW= I know. Inserting "happen" makes it sound more emphatic and it is used to mark an opposition with something someone said before (to express disagreement, contradiction, annoyance, etc.). For example:

- I think Susan is really stupid   - Well, she happens to be my sister so be careful with what you say.

- You’re wrong, Salisbury is in the county of Kent, not Wiltshire   - No, you’re wrong. I happen to be from Salisbury and I know perfectly well where my city is.


DRINK UP= To drink something up is to drink it completely, to the end.
- Let’s go to another bar
- Wait, I haven’t finished my beer
- Ok, drink up, it’s getting late


GOT MILK?= “Have you got milk?” or “Did you get milk?”. In this context it is probably the second meaning, which is the same as “did you drink milk?”, or in more standard British English: “have you drunk milk?”.