You need to cancel through the iPad general settings. Open up the settings for the iPad, scroll down until you see iTunes & App Store. Then, select the apple ID that you used to sign up for the subscription. When you get the pop up, select View Apple ID. Scroll down until you see subscriptions. Click on Subscriptions and it will show you your subscriptions. Find Tynker and select Cancel. This will set the subscription to terminate at the end of the current subscription period.