I created a registration form, but patron says they couldn’t submit two forms- one fir each child attending

I checked your form and I can see that the "Unique Submission" feature is set to "Check cookies only". This feature prevents multiple submission through the browser cookies. It means that the same user cannot submit multiple times using the same browser.

If you want to turn it off, then please set it to "No Check". For more details, please check this guide: https://www.jotform.com/help/64-How-to-Set-Unique-Submissions-on-a-Form.

Hope that helps. Thank you.