If the veracity of your claimed child care expenses becomes an issue, I doubt that your bank records would demonstrate that you were paying for child care (as opposed to simply paying for something). If you are paying somebody under the table, and have no contract, receipts, or other proof that you're paying for child care, that could be a problem for you if the claim is disputed.

If you're going to try to claim that you're paying somebody under the table, in an amount so large as to exceed what the trustee may view as reasonable for child care, that would trigger concerns of fraud -- and if you're paying at that level, the question is raised as to why those payments aren't being made to the best child care provider in your area as opposed to being paid under the table to somebody who doesn't even operate a licensed facility.