I would say something like... that's so sweet of you to offer, but I'm not ready for her to be without me right now. I'll definitely let you know when I'm ready!
That's something I used to say because I felt the same way.
I'm not sure I wouldn't let anyone watch my daugher till she was like 2 and they knew i was going though some ppd/anxiety bad so I wasnt ever "nice" or "mean" but idk just be like ummm no I'm good.