I suspect that Samantha has learned what her kids will eat, and only serves that - to the point that, in her mind, "they'll eat anything," because she never serves them stuff they won't eat.
The gagging is truly unacceptable. I was a picky eater myself, and would have been hung out to dry by my parents if I'd pulled something like that.
If the parents realized that the children were simply not willing or able to eat the food, the most they should have done would be to ask, as a great favour, if you might have some hotdogs or something to grill quickly, just so the party wasn't adversely affected by a bunch of hungry, cranky kids. Sniffing that you were rude not to have them in the first place is ridiculously rude, particularly when they told you in advance the menu was acceptable.