My Account/Settings & PINs/Payment PIN is your friend for the future, in case Live Chat doesn’t volunteer that information.
Also, as you will find, Day Passes can’t be cancelled - they just expire without renewing - though you might be able to get a refund. Keep plugging!
Firstly add a payment pin to your account by going to My Account > Settings & Pins section whilst you are logged in on here, which then should prevent this happening in the future.
Then jump over to Live Chat by opening the link page below, select the most appropriate help topic or nearest help topic to your problem, then once you reach the final stage scroll down towards the bottom of the screen page where you should see a Get in Touch option.
Click the drop down arrow below Get in Touch where a live chat button should hopefully appear on screen.
When you contact live chat explain to them whats happened and ask for a refund for the one day sports pass accidentally purchased by your little one.