Welcome to the NS Community!

Seems like you are trying to order Kids Vrij on your own card?

This needs to be a personal card of your child (age 4-11), which you can order separately for € 7,50

See https://www.ns.nl/en/season-tickets/kids-vrij.html for info. Unfortunately the ordering process is in Dutch only.

They have their own personal cards. when I follow the link, it asks me to login, I log in to my account, then it shows me my personal information. below that it asks the ov card number of my kid. then it goes to the payment page, and gives the error 

Good morning sep_a. That is strange. I would recommend to contact our Customer Service. My colleagues can take a look at the situation. You can reach them by phone (030-7515155), via Twitter and Facebook, or you can start a chat conversation (optin 'Chat with NS'). Do note that it is extremely busy today due to the storm, so it's probably wise to contact them later this weekend or next week.