Rise above it if you can. Vent on here if you can't. I had to (well, in my mind I had to) allow my MIL to take my dd for a day even though MIL drove me nuts and was SO controlling.
I put up with years of MIL putting dd in different clothes after I'd dropped her off then changing her back into her own clothes by the time I picked her up (only found out cos she took photos and showed me!) and bragging about the fact that dd was kissing her all the time (yes and she kissed the dogs a**e several times too, so that one didn't tick me off too much grandma)
And one day, your ds will grow up and tell you he loves you more than he loves his nanny. I know that's mean and low but my God, I loved it when my dd started saying that to me. Though I did, in my defence, simultaneously say that she must love nanny lots and lots as nanny loved her to the edge of the moon and back.
I look at it this way. One day with nanny = no child care fees to pay AND the added bonus of us not having to go over there and see her so often, because she's only interested in dd after all
Grandparents. Can't live with em and sadly, they're not around long enough for you to realise that actually, living without them isn't nearly so much fun.
Good luck with it all and don't beat yourself up too much if work seems a major struggle at first. You'll soon be juggling home, work and kids like a Billy Smarts Circus professional.