Make it clear that there is no need to lie. We all forget things, like washing our hands or cleaning up or homework. You do not mind if she forgets. This creates an environment where she will not be afraid of punishment or your disappointment if she needs a reminder.
Go with her to the bathroom. Let her know that hand washing is important and you trusted her to do it on her own before, but she lied. Now you will go with her and check on her until she has earned back your trust.
Do not create situations where she needs to lie. Kids do not want to disappoint you and they do not want to get in trouble, so, when asked if they did or did not do something they will lie. Stop asking. If you know she did not wash her hands, just tell her to go back and do it.
As for today. Go talk to her. Tell her that it upsets you that you cannot trust her. If she needed money or wanted to buy something, she just needs to talk to you about it and you can work something out. She can earn or save for whatever it is that she needs. Let her know that if the lying continues she will loose some independent privileges since you cannot trust her to make positive choices when left alone. If you are cooking in the kitchen, she will have to read or color in the kitchen. If you are working on the computer, she will have to bring something quite to do next to you. Kids do not want to go backwards, the want to keep gaining privileges.
Lastly, take note of how often you, and other grown ups in her life lie. If someone calls and you do not want to talk, do you tell the person who answered to say that you are not home? Do you say that you have plans (which you do not) because you do not want to hang out with someone? Do you tell someone that you like a gift, then say you hate it when they are not around. Kids pick up on that stuff.
I would love to hear how it all turns out,
B.
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