Bastiaan Anthonie otf Baaij There's currently no way to open a sub-account where you can only deposit money but not withdraw. The app makes it a little bit more cumbersome to withdraw from the "savings goal" accounts, but under the hood they work exactly the same as regular sub-accounts. And while you could also open a Joint account together with your child, you would still have full access to it and could theoretically send the money to another account whenever you want.
What you can do is "hide" an account from your Home tab overview by opening the Settings ⚙️ on that screen, selecting "Visible accounts" and untoggling your savings account. Then it would be psychologically easier to forget about the money stored on that account.
Or maybe someone else from the community has a good idea here :)