Welcome to the forum. You can cancel your pass by going to your My Account > My Passes page http://account.nowtv.com/passes
You should see the kids pass you have on your account, with a cancel button next to it. Just click cancel and follow the steps all the way to the end.
As for a refund, you can try talking to live chat, explaining the situation and seeing if they can do anything for you as a courtesy. To get there go to the following help page
I'd also recommend setting up a payment PIN on your account to make sure your children don't accidentally purchase any passes, and there’s also a parental PIN you can set to restrict what certificate content they can watch should that be something you are interested in.