Disclaimer - I don’t have kids, the closest being my niece and nephew who I see once a year roughly. So my perspective will be as an adult who may be eating at a restaurant where others have kids.
From the restaurant’s POV, most of the stuff they offer on the traditional kid’s menu is dirt cheap to make/offer, and even at kid’s menu prices, they probably still profit (looking at fries and chicken fingers especially that are probably mass market frozen options tossed in a fryer briefly). So low effort, medium profit, and not much space in freezers for the just in case - not something they worry about spoilage/turnover losses.
As @kenobi_65 pointed out, successful restaurants are making decisions based on their market, and they aren’t fools, so I seriously doubt they are being harmed, and if the math changes, they’ll phase it out for “lighter fare” or half-sized portions, which inevitably are about 75% of the main price (touched on by @Ann_Hedonia).
From my, or my fellow diner’s childless POV, if it keeps your kids happy, along with your old cellphone or their tablet, I’m all for it. Much better than having freaking out crying kids spoiling our rare meal out.
I’m 100% onboard with the adults offering (sometimes semi-forcibly) a child a taste of their own entree. That’s how I learned which dishes I might get more adventurous on as I grew up, and while I wasn’t the picky eater of some reports, I wasn’t adventurous with foods until high school.
Last, I’m not going to rag on the kids too much no matter what. I, along with most people I know are going to select from a smallish selection of “I’ll probably like this because I like that” selections MOST of the time when I’m going out. Especially at slightly upscale (in price at least) eating out, few people can tolerate rolling the dice on complete unknowns on a menu once you’re throwing down $30-40 a plate for food.
If someone else is paying, or I’m going with a friend who knows my tastes, that’s when I’ll experiment when I’m eating out. Otherwise, there’s tons of stuff I want to try, but that’s for making at home. Some stuff turns out great from the get-go (my recent shrimp and grits), some is fine but is going to need tinkering to suit my tastes, and some is fine for the experiment but not worth doing again, and of course, some is a disaster that you eat a few bites of to make sure and make something fast from the freezer because it was a colossal failure (in taste, preparation, or mis-match of preferences).