This type of theft has to be stopped. Thankfully to date I haven't had anything of this type with my grandchildren but, should it happen, I'd :

1. Talk to the child's parents and expect them to take appropriate action. If it was a major theft, I'd also tell them I will be informing the police.

2. Advise the police and ask them to talk to the grandchild and tell him/her of possible consequences.

3. Write him/her out of the will if there is no remorse and tell the family what it is all about. If the parents consider it a playful prank, write them out of the will too.