What happens if I wasn't eligible for the tax free childcare?

In my job I have the potential to earn a bonus each year. I don't know if I will get £0 or a few thousand. This bonus has the potential to put my annual income over £100k. I understand that I can say in the tax free childcare quarterly confirmation 'No I don't expect to earn over 100k' until I know that I am/am not getting the bonus. But then if I get it and it goes over 100k then Im no longer eligible for that tax year. But is there a scenario where I am no longer eligible and I have to pay it back? If I get the bonus in march, I would have had a whole tax year of tax-free childcare. But if I get it in april, I have a whole year without tax-free childcare, even though I actually earn the same in both years? It doesn't make sense to me. When and how is the eligibility checked, and what happens if you had support that in the end it turned out you weren't eligible for?

Hi,
For the best advice here, please contact our Child Benefit department directly by webchat or phone, via Child Benefit: general enquiries.
Thank you.