Paul
Group account is the exact thing you need as this is a legal joint account. Set it up with 3 members and you, your wife and a kid are all 3 full owners. Repeat this 5 times.
Note: a group account cannot change accountholders while the account is open. It must have the same owners from opening till closing.
Before inviting someone they must already have a bunq account. You can even invite free accounts as long as you are Premium, but then they don't have cards or other Premium functions.