I posted quite a bit of information in the thread a couple of posts down. In general, the team members are pretty good about sizing people up and placing them in the cars in such a way to give them the best chance of riding. That being said, in general, the main test seats for most of the rides are right out in the open and often have little kids sitting in them. Forbidden Journey has a second test seat inside the castle near the sorting hat.
If someones a little embarassed about using the test seat, you can combine it with a photo OP. A lot of people take pictures of their friends (yes it's especially kids that are too small to ride) sitting in the test seats. But if someone is getting a camera out for a picture it looks like you taking a picture instead of seeing if you fit.