And I still don't understand the outrage. what is the difference between feeding the child a few bits of food from your plate, and feeding them a small pot of food that you have brought with you. The restaurant is still getting the same amount of income and the child has a more appropriate meal.
The difference is the same as not eating a starter or dessert vs. bringing your own starter and dessert. Also, I would always ask if it was okay first and they would usually bring me an extra plate if it was. The cost to the restaurant is the same but you are not using the restaurants facilities to eat your own food without their permission.
If there is something suitable then buy it, but if there isn't, taking your own is fine. I regularly did it and was never challenged.
It really isn't. It is very rude. You could look at the menu online or before going in and if there wasn't anything acceptable to eat you should have asked the restaurant in advance if it was okay to bring food from home or gone to a different restaurant.
Every time I went out to dinner with my autistic son, up to the age of about 10, I took a drink for him. He had an extremely limited range of food and drink. We could always find something on the menu that could be adapted for him, but if they didn't stock his drink, he wouldn't drink and would be very distressed. We ordered 4 meals, 3 drinks and an empty glass please - never once did they ever so much as raise an eyebrow.
That's fine if you asked them and explained the problem. If not, I don't think it would be at all acceptable to take a 10 year old to a restaurant, use their facilities but give him food you had brought from home.