S.A.
If you can't shake off that uneasy feeling, your intuition is definitely telling you something. If I were you, I would let her go because the trust is gone. Even if you were to ask if it was her who put the gum there, she will most likely deny it. If you are worried about finding another babysitter, you shouldn't. There are a lot of reliable people out there, you just need to do a little bit of digging. Hopefully someone on here can lead you to a nice person. But trust your gut, it's telling you something. Good luck and let me know how things go.