Which Curve card do you have Nathan? The original beta version was not accepted as it was not recognised as a debit card, but I have used the new Mastercard version to pay my tax on the Gov.uk site so it is not a site issue.
Out of interest is this with an Amex card underpinning it Nathan (I see you are a beta tester.)
It is a new card. I also use it to pay HMRC. It is specifically the childcare account that doesn’t work. It didn’t work prior to being a beta tester either.
This should be possible as long as you are using a debit card as a funding card and you have the Curve Debit card. Have you had the chance to contact gov.uk as ask them why it hasn’t been possible to pay the childcare scheme specifically?