It is a tough thing to decline an invite. DD1 really does not get that many, but I did have one that I would not let her go to. The birthday girl as a frenemy to her and had caused way to many tears in this house. I declined the invite and I lied. Just told the mum we were away that weekend. Felt bad, but I would have felt worse being at their house wanting to have words with parents about their daughters behaviour.
Been at a HJ's party, very similar to the one you talked about. Really not fun at all, very stressful to be at and honestly the parent holding it had no clue that there was anything wrong with the place. It would have gone completely over her head if someone had declined an invite due to personal ethics. We do eat junk food and do eat HJ's, but I would be very inclined to decline another invite to a party there.
So I would say, decline but just say you already had plans and then take the family on a nice day trip somewhere.

