Your daycare woman sounds as habitual as your daughter, but her habit is to "correct." I'm sure different moms will have different takes on what sorts of correction are okay. For me, this is not a caring or respectful way to treat a child, and I would care very much if this were happening even during the day when I wasn't there.
It seems quite reasonable, to me, to ask a caretaker to change a habitual response, not by just telling her not to do it, but by acknowledging the value of what she's trying to accomplish, telling her how you feel when you see it happening, and suggesting an alternative action.
For example, "Miss Jean, I want you to know I appreciate your willingness to help Suzi stop thumbsucking. I feel upset when you slap her hand away, though, because to me, that's just not necessary or respectful. I would like you to just take her hand away from her mouth instead of slapping it away. Will you please make an effort to do it that way in the future?"
If she's a smart lady, she will at least never let you see her do that to your child again.