Stop it.
They asked for an "average" weekly cost. That means working out what it is likely to be over a period of time. You anticipated you'd be using the child carer but sometimes things changed. That is completely understandable, especially in light of your mum's operation and her needing some company to cheer her up.
Do sit down though, quietly with a cup of tea, and cast your mind back and try to remember what you included/didn't include when you were doing the first estimated figures just in case you suddenly remember something. Did you, for example, include travelling costs to the childminder when you worked it out? Or anything like that?
Truly, the sums involved here are a drop in the ocean to HMRC. Their only interest will be in recovering any overpayment.
DDx