Hey OP, I have 15 yr old twins, both autistic. They have never been abroad and haven't actually even been to a British beach for years.
I've talked to them both several times this summer and they're emphatic that they have no interest in either. I might try and get them to a beach during the quieter seasons, but honestly, I think we'd drive there just to spend five minutes and they'd be done! I just don't think it's their thing.
Same with travel. Zero interest. They find any kind of travel really stressful. They're both just happy pottering around close to home. Maybe someday that will change, maybe it won't.
I'm also autistic and ADHD, and honestly, I think travel is vastly overrated. There are elements of going on holiday overseas that are very nice, but for me, the good bits just don't make the rest of it worthwhile. I've done plenty of travelling in my lifetime, mainly because I thought I should, and I can't say I ever especially enjoyed it.
In the modern world, and especially on sites like MN, you can feel like a failure if you're not constantly seeking to broaden your horizons and visit new places. But I just think that's not for everyone, and that's OK.
Maybe have a conversation with your older child and see if it's something they're keen to experience. If so, maybe a day trip or something to France as a starter? But don't make them feel as if it's something they HAVE to do, or that they're a failure if they have no interest in it.
Editing to add: there's a good MN phrase that I really like: comparison is the thief of joy. You sound like you're giving your DC a lovely childhood, don't compare with others 💐