What you don't know won't hurt you
what (someone) doesn't know won't hurt them
Remaining ignorant or uninformed about something will allow someone to not have to worry about, feel responsible for, or get upset by it. A: "Where are you going?" B: "It's better that you stay in the dark about this. What you don't know won't hurt you." A: "Mom will freak out if she finds out that you took the car without asking!" B: "Of course she will. But what Mom doesn't know won't hurt her."
What you don't know won't hurt you.and What you don't know can't hurt you.
Prov. If you do not know about a problem or a misdeed, you will not be able to make yourself unhappy by worrying about it. (Often used to justify not telling someone about a problem or misdeed.) Ellen: What a beautiful diamond necklace! Thank you! But how on earth did you get the money to pay for it? Fred: What you don't know won't hurt you.