I'm sorry you're going through this. I'm not going to advocate speaking with your bf about this. He's not helping the problem but your issue is with them, take it to them.
The truth is that all older adults feel like they have a right to run our lives. The chances of that double when we have kids and quadruples if you ever need their help.
I lived in a relationship like this during Highschool and the next few years after. I finally couldn't take it and after the years of saying, I'm an adult and this is my child, well I asked for a divorce and was took to the cleaners, they even took my child.
When my current husband's family tried the same thing, I put them in their place. Me, not my husband, told them point blank, if you expect any kind of respect from me then you need to respect my rules and guidelines, period. After 10 years of very limited contact they get the point. But we had to move out of their neighborhood to achieve it. (our main problem is that the younger siblings still live at home and have drugs at the house, we're not the only kids that have made this rule.)
Like you were told before, pick your battles. Candy and pop are part of being a doting grandparent. Make sure you get the kids to brush more often on those days. Make sure the kids are getting lots of floridated water too. I know the tap water around Pawnee can be kinda iffy and bottled water is great but it lacks the floride that they could get from tap water.
Perhaps it would be best to cancel this vacation until you have better boundaries. You don't want to send mixed messages to them or your kids. Like, I don't want you to disrespect me in front of my kids but I'm willing to take it in order to get to go on vacation, uh, not in my lifetime.
To get through this you could adopt 5 ideas from a book on Oprah's Best you you can be reading list: The 4 Agreements, Toltec Wisdom by Don Miguel Ruiz
~It's not what people say about/to you it's the way you take it
~Don't take anything personally
~Don't make assumptions
~Be impeccable in your word
~Do the best you can
For instance, if you don't want them participating in school functions then don't tell them about the function.
If you tell them to myob, then don't go to them with any problems.
Personally, I think there's something about that area that just makes people nuts. I lived in Cleveland for 5 years and the level of actual adult behavior up there was much more questionable than I have seen anywhere else in the state.