A nanny in the case meaning?

Hi, What is the meaning of "a nanny in the case" and" you need it" in this text?

" We thought the last time you said, 'bring these discovery things back to me and I will be a nanny in the case'"

"I now find myself up to my ears,"said the judge. "Trials stacked up back to back. I don't think I can perform the nanny function. I'm sorry, you need it, "he added.

Thanks