Multiple homes

My boys (grandsons whom we are raising 8 yrs old) go to their Nanas once a week who is also trying to implement self government with them. A few times it seems the corrections are left for us to do at home. Often given a list of wrong behaviors requiring attention. I have expressed that they should be done there and that I don't need to know everything, unless harmful. I have also expressed that anything she has to tell me shouldn't be in front of the boys. Am I handling this correctly? One of the boys seems to be exposing himself every now and again. This time at Nanas. He hasn't done it at home for over a year. How does one handle this? He thinks it's funny but I have now pretaught and let him know that he will receive a major correction if this happens again. He says ok and again this hant happened at home for over a year. Help