I don't really know why you're asking, since they aren't your kids and I don't see calling up your cousin and dictating what groceries they should buy turning out well. Valid question, though.
But my kids constantly snack, and where they do occasionally have the unhealthy stuff, I mainly try to keep around healthy snacks and there are tons of good options. Cheese cubes, string cheese sticks, yogurt (greek yogurt even better), fresh fruit, favorites around here are always apples, bananas, and berries (also sometimes oranges, kiwi, and others). Veggies are always good too, popular around here are celery, carrots (small and large), tomatoes, and cucumber. If your kids aren't big on veggies, you can make them more appealing by accompanying them with veggie dip. You can buy them premade, or you can go even healthier and make your own by using fat free sour cream and mixing in a packet of ranch seasoning. Same goes with fruit, they sell fruit dip, and you can just use flavored greek yogurt, super healthy. You can make your own zucchini chips in the oven super healthy, hard boiled eggs, beef or turkey jerky, peanut butter or cream cheese or hummus on crackers, fruit smoothies, freeze dried fruit is yummy and easy and stays good long (banana chips are my favorite lol), nuts like peanuts and cashews and mixed and my kids LOVE sunflower seeds.
For small easy meal type things I like to keep around healthy sandwich and wrap materials, salad mixes, breakfast burritos that I make myself and then put in the freezer. There really is a lot of easy stuff to keep around, to keep from eating junk. I try to keep healthy home made stuff around too when I can, like oatmeal muffins and cookies etc.
If they must have chips, I try to go with the baked ones or gold fish they are a little bit better but not much lol. They really like the Nature Valley granola bars too.
Don't get me wrong though, my kids do occasionally get junk. I'm not a health nut nazi, but it's definitely not a regular thing around here at all.
