Put it on the Home Screen: in Safari tap Share → "Add to Home Screen". It then opens full-screen with no browser bar.

Lock the iPad into the game: Settings → Accessibility → Guided Access → on. Then triple-click the top button while the game is open.

No sound? Turn up the volume and check the mute switch in Control Center. Sound starts after the first tap.

To reopen this panel, hold a finger on the top-left corner for 2 seconds.