If you choose not to subscribe, you will not have access to most features in the app. You can of course still use the app, but to a limited extent.
The features that work without a subscription are:
Instead of savings goals, you can create wishes (not as interactive)
With a subscription you also get:
The ability to connect a bank account to get the most out of the app
Quiz about money
Evaluations of transactions
If you choose not to upgrade, your existing savings goal will be converted to a wish after the transition period. The bonus will be reset to zero and any bank account that has been connected will be disconnected. Earned stars remain for the children.