I have never had any difficulty in keeping secrets.
In the case of a child being harmed, I would tell the child that no-one child nor adult is bound by a promise they have been forced to give, and that actually, if we know a crime is being committed we are obliged to report it to the police.
I cannot imagine that I would not report a serious crime that an adult told me about, and I might well in the interests of my own safety not tell them that I would need to report it.
In the case of someone having helped themselves from a cash register, or shop-lifted, I would try to make them see that they needed to stop and replace the money they had taken, or send it anonymously to whereever they had stolen money or goods.