You can cancel a plan by turning off automatic renewal in settings via your Parent Dashboard.
To cancel your subscription on iPad, 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 Subscription.