Must I send my children to visit their grandfather, who caused a financial problem and breached trust, or should I prevent them from visiting him to protect them?

Part of good marital relations is for each spouse to treat the other's family well and to help their partner honor their parents and maintain ties of kinship, overlooking their mistakes. This fosters affection. It is not permissible to prevent children from visiting their grandfather, as this severs kinship ties, unless there is a fear of corruption affecting them from such a visit. In that case, it is permissible to prevent the visit to the extent that the harm is removed.