There's no reason why you have to tell them it's for you
I ordered from th kids menu several times (at the counter sevice restaurant at the land in Epcot the kids meal was really good). I was visiting solo so I only ordered the kids meal,there no adult meal accompanying it. I was even able to substitute coffee instead of juice, no even batted an eye.
I guess if they ask you can always say that your ordering it for your kid, but I doubt they would.
I've never seen anybody having a problem ordering a kids meal at a CS place. I'm a picky eater, and some places the kids meal looks the most appetizing.
I used to do this at CRT all the time until the last time I was there and decided to order off the adult menu and the CM always said sure no problems! They had no problems having me and my sister who at the time were both adults ordering off the kids menu.