Well, firstly they are just at the age for finding such family gatherings a bit boring, especially if there are younger kids present and no cousins their own age. I remember having a few years where I turned down invites to family gatherings, or only went for a short time to be polite, as I just wanted to spend time with people my own age. It’s a normal developmental stage, I think, when you’re still seen as not quite a fully fledged adult by wider family. By early to mid 20s I was happy to attend family events again, as an adult and an equal. If your relatives’ children are only young they won’t understand this as they’re in thr middle of “our kids come everywhere with us” stage.
I would not be making your children attend things they don’t want to attend unless important like a wedding or funeral or someone’s big birthday. (Where it’s usually easy to find a quiet spot or other relatives a similar age). They do need to learn that some events are just non negotiable in families and unless there is a huge rift they need to learn to suck it up and show up.
I also think they are old enough to be standing up for themselves regarding the comments, with you as their “cheerleader”, on their side and modelling ways to respond eg you should laugh and say “oh, FIL, no-one wears X type of shoes anymore, haven’t you noticed all the young ones wearing Y? Isn’t that right, DD? What’s the name of that tik toker again, who started that trend off?”
or, alternatively, “yeah, DD finds the current trend of everyone wearing X item of clothes a bit boring, it’s no fun to look like a clone of everyone else, is it? They prefer this look as it fits in with their musical tastes, doesn’t it, DD? Tell Uncle Pete what the name of that band is that you like, to see if he’s heard of them.”
you want to make the point to the relatives that there is a generation gap there and by constantly commenting in what I assume is a disparaging way about their outfits they’re just showing up how old fashioned and stuck in their ways they are! They’ll soon shut up when they’re made to feel past it 😆