Some crackers have whole grain versions that taste as good or better than their white flour versions. Goldfish and Cheez-its are the two that come to mind. This doesn’t make them “healthy”, but at least there’s some better fiber in there. Compare nutrition labels.
Also try veggie chips/sticks. Again, they’re not “healthy”, but… there’s some better stuff in them than normal junk food. They are junk food with a small amount of bonus nutrition.
Freeze-dried fruit is addicting, but expensive. Try it and/or dehydrated fruit. The latter is chewy while freeze-dried is crunchy and melts in your mouth.
You can also try making your own nuggets or tenders with whole-grain panko breadcrumbs, and seasoning. The trick is to make it better than McDonald’s, which is easy for an adult, but tricky for kids. I don’t know the magic sauce though.
One of the latest Splendid Table podcasts had a guest on to talk about cooking healthy for kids (or she just wrote a book about it, I didn’t listen yet.) It’s a great show. Check that out.