Klaus I've done this in The Netherlands, so perhaps your experience is different.
You don't need a smartphone, but you do need a phonenumber whcih can receive SMS. You can setup the accounts on your own phone, but you'll need to enter the code that you receive via SMS.
So you can manage their accounts on your phone. Note that you also need a bank account that is not bunq. You need to make a transfer using iDEAL or Sofort and you can't use any bunq account for that. I don't know why this is. I'm still planning to make a post about it but thinking about it still makes me angry so I haven't yet.
In the past you could create a Parent-Child connect from the child bank account to your own. I think this got removed in the last update. That connect can only be stopped if both parties agree (while other connects can just be broken by the owner of the account). I think you can still open new bank accounts in the bunq app though. Those don't get the same connect automatically. Also this only makes it possible for you to see their accounts (and pay from it) but it doesn't stop them from spending it in a shop or online. Of course if they don't have access to a bunq app of themselves they can't change the limits you put on the accounts and these are still enforced.
You can add other bunq accounts in the app by pressing on the profile button, then in the top the icon with two persons and then the big blue plus sign at the bottom. Or you can long-press the profile button, choose "more options" and then the same big blue button. The long press is easier because you can also use it to switch accounts (aftert hey are added on your phone).
Only when the new accounts of your children are active you can invite them for the bunq Pack. You send the invitation from your own account, then switch to their account and accept the invitation. This is also how it works for connects. You do this in one account and then switch to the other account to accept.
The API key stays valid untill you revoke them as far as I know. If you give them an API key and bunqDesktop they can still change a lot of things. You have to use OAuth so they can only read things. You can read more about it here: https://together.bunq.com/d/5763-bunqdesktop-the-unofficial-free-and-open-source-desktop-application-for-bunq
Transfers from bunq to bunq are always instant so if they urgently need more money to pay something you can send it from one of your own accounts right away.
