it is not for GPs to interfere in how the parents bring up their own children, including discipline.
this lack of respect to the parents' authority over the children is one of the main tensions that can lead to low/no contact.
so by being too bossy, interfering the GPs shoot themselves in the foot, as they are then denied any access to GC.
also the custodial rights of parents with shared responsibility, means children can move between both households. this does not extend to any other households inc GPs. so it is quite black and white in that regard.
just because GPs want like to see GC, does not create a right.