Nabi Big Tab is a 24 inch screen tablet for under $400.
More at: https://www.nabitablet.com/nabi-big-tab-hd24
The screen is mounted on a standup frame, that is on the heavy side, that still is mountable on a wall.
The Big Tab runs a kid friendly Android that can be unlocked into Mommy mode for running standard Android aps.
The downsides we are still exploring are: (1) it always starts in kid mode, (2) how to tag aps to run in Kids mode, (3) how to disable the wake up on local activities. None of these are an issue for a public space, they are annoying when used at home.
There is also some question as to production availability – you need to go to NABI to determine current production.