The subscription for Pinwheel covers the software on both the child's device and the Caregiver Portal. (more info here on what's included : What is Included in the $14.99 Per Month Subscription?)
The software and the hardware go hand in hand.
If you were to cancel the subscription then the software on the device would deactivate and the phone would be essentially useless.
We find that when your kid is ready to "graduate" to a normal, unrestricted phone then you can pass the Pinwheel down to a younger sibling, cousin or even friend. One phone to rule them all.
If the case be that you want to cancel your subscription, you can do so here.