Ensure "local server multiplayer" is enabled in options.
Ensure none of the devices is using a VPN.
Ensure ALL of the devices have the latest version of Minecraft PE. If that's not so, there's no way it's going to work.
To update an app on iOS:
- Open App Store app from home screen.
- Browse into updates tab.
- Scroll down to find Minecraft PE.
- Press the "UPDATE" button next to the game.
- If needed insert Apple ID password.
- Wait for update to finish.
Repeat this for all iPads.
Ensure all the iPads have A DIFFERENT MINECRAFT NAME!
You can change the name in Options menu, from "Steve" to something else.
If it still doesn't work, this might be a issue with Unique Device Identifier of Minecraft. Reset it by uninstalling and reinstalling Minecraft PE. This will unfortunately remove the worlds.