I don't know where you live, but I would be very, very surprised if your country happened to have "mobile OS" enshrined in law as a forbidden avenue of price discrimination.
From a quick check on my government's website:
- prices must be displayed outside and inside the restaurant
- the prices can't be different depending on where they're displayed
- the displayed price must include taxes and service fees
That's enough to guarantee there's no possibly legal price discrimination. Not a single mention of "mobile OS".
Can they refuse to allow an adult to order from it?
Actually children menus are sometimes marked as < 12 years old now that I think of it. Not sure if it’s legal, but I don’t think it would be enforced.