Yeah - OPs AH-ery isn't that he won't require his kid to drink the milk regardless of what his kid actually wants (and to be fair to the kid - I'm not a big fan of milk in general so cereal milk makes my stomach chrun). OPs AH-ery is that instead of being a parent and talking to this adult on his son's behalf to come to an agreement ahead of time, he told his 10 year old to go to someone else's house (someone who is at least temporarily responsible for him) and be disobedient and uncooperative.

Do I think the kid should be able to refuse? Yes, absolutely. There are lots of new situations that could arise and the kid needs to know it's okay to refuse to do things. At the same time, this one is a known rule. The proper thing to do is address it with the parent yourself first and then give your kid permission to refuse if necessary.