My family and in-laws are pretty much the same way. I got into an argument with my mother about birthday cake, as well. These are people who constantly say things like, "We're not dessert people," but then insisted on buying a grocery store cake that no one wanted for a communal birthday gathering when my LO was 10 months -- they only bought the cake, I'm convinced, for my baby to eat. I said for months that she wasn't eating any cake and they would argue and argue until I'd yell at them, then they'd make a big deal about my yelling at them. What's the quote given to students of Zen, "If you think you're so enlightened, go spend a week with your parents"?
My DH is the same way and never really says anything to his parents about things that bother me. I always have to tell him what to do ahead of time. Before you go, you can say, "If your parents start talking about food, this is what you need to do." If he's like my DH, he'll agree but then back down when the time comes, but then you can give him angry looks that may remind him. Also, if there's things you think they'll feed your LO, take a healthier version. Like for my DD's birthday, I made her healthy cupcakes instead of a cake. And we always have frozen banana puree for her instead of ice cream.