@desur

Welcome to the Netherlands!
And welcome to the KPN forum!

KPN does not have specific family subscriptions for mobile, but you can share data with each other.
If you and your wife take out KPN Unlimited data & calling, you will each have 50 GB to share with your three children.
Your three children can then take out a KPN subscription with 0GB or 3 GB of data and unlimited calling, which you and your wife can then supplement with MBs from both your 50 GB bundle.

All family members (you, your wife and three children) must be within the same "Mijn KPN account", so that you can share MBs with each other. You can only arrange this via the "Mijn KPN app" from the App Store ( iPhone) or Google Play (Android).
So the entire family within one account!

Hopefully I have given you the necessary information.
Otherwise, I would like to hear from you, because we are happy to help you on this forum.

@desur

Here is some additional information about how you can share the MBs with each other:

Multiple KPN Mobile subscriptions at the same address? Easily share MBs with each other in the MijnKPN app.

Share 50 GB / month with multiple KPN mobile subscriptions at one address
Save on your mobile costs together with your partner, child or housemate
Take full advantage of a 0 GB subscription by sharing data
Automatically share your MBs every month

Automatically share MBs

With automatic MB sharing, you don't have to do anything after setting it up. The app then shares your MBs every month with the indicated number when your new monthly bundle starts again.

How do I automatically share MBs?

Log in to the MijnKPN app
On the mobile tab, click 'Share MBs'
Select 'Automatic' at the top
Choose the amount of MBs and the person (number) you want to share the MBs with
Click 'Save'. Your MBs are automatically shared when you start your new monthly bundle again (usually the 1st of the month) at 7:30 AM. You will receive a notification if it was successful
Don't see another phone number to share? Then add this first.

Share MBs one time

Do you want to share MBs with someone once?

Log in to the MijnKPN app
On the mobile tab, click 'Share MBs'
Click on 'Share MBs once' at the bottom
Choose the amount of MBs and the person (number) you want to share the MBs with
Click 'Share'. The MBs are then shared equally
Don't see another phone number to share? Then add this first.