Go to the Photos app. Tap the album tab. Then tap the Camera Roll album. Tap the edit button in the upper right corner. Tap on all of the photos that you want to delete in order to selecte them. The red delete button should become active.
If that is what you are doing and you are not getting the expected results. Quit the photos app and reboot your iPad.
Go to the home screen first by tapping the home button. Double tap the home button and the recents tray will appear with all of your recent apps displayed at the bottom. Tap and hold down on any app icon until it begins to wiggle. Tap the minus (-) sign in the upper left corner of the app that you want to close. Tap the home button or anywhere above the task bar.
Reboot the iPad by holding down on the sleep and home buttons at the same time for about 10-15 seconds until the Apple Logo appears - ignore the red slider if it appears on the screen - let go of the buttons. Let the iPad start up.