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 the app you are looking for and select cancel.
You may also find some other subscriptions you forgot you were signed up for, or gave up searching for, while you're at it.
It is embedded deeply within links with no intuitive or well marked path. Also, at least with the subscriptions that I've tried to cancel in the past, they don't tell you how to do this online--you have to email them and ask how to do it. (Looking at you Tynker and Minecraft.)