Um, I work for Pizza Hut, I have a degree, it was my job to see me through uni, and I work very hard for not very much money. Like any workplace that doesn't pay very well there are some people working from the company who aren't terribly bright, but It is a gross generalisation to say that Pizza Hut staff tend not to be "the sharpest pencil in the tin."
The branch I work at allows sharing to a degree, I wouldn't allow 15 people to share one salad bowl and a small pizza or one refillable drink. Some people really try it on! We don't charge babies and toddlers to eat from the buffet though, but that is at our own discretion and depends on how nice the customers are! Obviously if there was say, one adult, 3 toddlers we would charge, but maybe for one child buffet instead of 3.
For the diabetic wanting to only pay for a kids buffet, you must be having a laugh! If you can't eat more than two slices of pizza at a time, what are you doing at a buffet? order something that suits you from the menu! You may not like the company, I don't like it much either but we have to make money or we do not get the hours we need to staff the restaurant properly and it becomes a viscous cycle of losing money through making too many concessions and unhappy, very tired staff having to serve more customers than they can physically cope with. So many people come in trying to fleece as much as possible out of our restaurant without thinking about the effect it has on our livelihood, there can't be a separate rule just for one customer.
To the OP, sharing is definitely expected and I very much doubt that the waitress will have gone quiet because you asked for an extra plate. It is not unusual for two adults to share a regular sized pizza in ANY of the stores in the whole of the UK, no one would raise an eyebrow at sharing a regular pizza and some salad with a child. If you wanted to share an adult refillable drink it might have been a different matter though. If she didn't even say anything about it there could be a hundred other reasons for her to be a little bit quiet, maybe she was embarrassed that she didn't notice your son was there? Maybe she was having a hard day or had forgotten something or was going to suggest something then decided it was actually a bad idea so didn't? You wouldn't be unreasonable if she had actually refused you a plate to share the food with, but she didn't! You're over thinking things, next time you don't need to try and justify sharing to the staff, just relax!