It seems strange to me that a couple of answers to this question seem to blame henry the grandparent for having cash in his own house. If you cannot find somewhere in your own house to put a bit of cash, without a member of the family stealing it, then your house is not your own. These two grandsons need to be spoken to quite firmly, and a threat made to tell their parents. This may or may not cause them some worry. Some kids couldn't care less if their parents are involved or not, while for others it would be the last thing they would want. Because they are your grandchildren, with easy access to your house, it is maybe a bit over the top to set up a camera, it should be possible to catch them out just by being observant. At the age they are, they probably don't appreciate the seriousness of what they are doing. They would probably be less likely to do it if it was anyone else other than their own grandpa. Maybe they see it as "not really stealing" because it is their grandpa. They do need to have it pointed out that this IS stealing, it is serious, and it must stop at once.