The short answer is yes. We have done this before, but our case may be a little different than yours. Our sitter traveled with us and we got her an extra key to our room - no tickets or credit card attached (she had a separate room with her own key, tix, etc., the extra key just made babysitting a lot easier!). We know our sitter very well and she was with us the entire time, plus if she had stayed in our room (though she didn't) we were in a situation where we wouldn't have incurred the "extra adult" charge.
It sounds like you are using a sitting service? I guess it's just me, but I would be wary of giving someone I'm not familiar with an extra room key. I also wouldn't give them my child's KTTW just in case it got lost. I think I would ask the front desk if you could have the extra key and then have it deactivated once the sitter wasn't working with you anymore.