My advice is to start of with just a quick phone call. It may be just that she forgot to give you the check, she forgot her checkbook at home or that she didn't get her check deposited (if she doesn't have direct deposit). I would give her a call and say "Hey, I realized after you left yesterday that you forgot to give me a check for the childcare. I was hoping to pick up a few things at the store (whether groceries or whatever OR alternatively mail a check for a bill) this weekend. Can I stop by and pick it up so I can deposit it on my way to the store?"
This opens it up in a nice way and puts it on her to explain why she hasn't given you the check if it was intentional. It also allows her to broach the subject easier if it was an accident knowing that you are open to listen to an apology and not harboring ill feelings if it was an accident.
Where you would go if it was intentional is kind of up to you. But this approach would put it out there in a way that is not judgmental and allows for further discussion.
I know (from personal experience) dealing with friends or family in an employee fashion can be kind of tough and can sometimes force you to choose between forcing an issue with receiving what you are due or letting it go for the sake of family/friendship. However, you won't know if you are in that type of situation for sure until you know why she hasn't given you the money and whether or not she IS going to give you the money.
Good luck. I hope it was just overlooked/forgotten by her and not a true problem.