Don't Make This Move When Ordering Your Child's Meal. Servers Hate It
As a parent, you probably think you're being really clever asking for an adult entree "just smaller, please" when ordering for your child. For the people on the floor, though, this request is one of the fastest ways to annoy your server. As someone who spent time both as a server and running a food business, I know that there is a rigorous system in place, and if that system already has a kids' menu, it's been thought out in terms of portions, prices, and preparation. So when you ask for a custom half-portion of a dish not listed under "kids," you throw the entire system off.
In most restaurants, portions are built around set food costs, so it isn't that easy to simply cut dishes in half. The pasta portion has probably been weighed out, the sauce may already be prepped in batches, and the protein could have been pre-cooked into specific amounts. So that smaller plate may actually cost the restaurant the same to make as a full one, but you're asking for it for less money. This puts the server in the awkward position of having to explain these kitchen economics, or they may say yes out of politeness and end up being told off by management.
Of course, children can be picky, and sometimes there's not a lot on the kids' menu that suits them. But before asking for a custom mini-version of their favorite entree, remember that this is not the easy request it sounds like it is. There are already lots of things customers do that stress servers out (including the rude way that customers order drinks), and being a server is difficult enough, so don't add to it when possible.
Other ways to order for your child
If your child isn't thrilled with the choices on the kids' menu, there are better ways to work around that, like ordering something off the appetizer list. These small plates may be portioned closer to what a kid can finish. Alternatively, you could share part of your own meal by asking the server to bring you an extra plate. Just consider how you time your requests, as servers are usually juggling multiple tables, and nothing slows them down like a guest who keeps flagging them down for extras one at a time. This is called one-timing your server, and to avoid being that person, order everything upfront.
It can be tempting to think of restaurants as places that should cater to every possible request, thinking that you're paying for a meal and therefore should be able to modify it to your heart's content. But the reality is that food establishments operate like a well-rehearsed routine. The menus have been planned long in advance, and there are prep lists and pre-portioned cooking that have gone on behind the scenes to ensure speed and consistency during busy hours. It's good practice to check the kids' menu ahead if you have a particularly fussy child, and you may want to check out the official Chowhound ranking of chain restaurants' best kids' menus.