I don't want my girlfriend's dad to meet our children.
Why don't you want him to meet your children?
He's too irresponsible, he would be a bad influence on them.
He is part of the family and has a right to meet the children. Let him meet his grandchildren.
You don't know him and the things he's done.
Well, you know him better than I do. If he molested you, you're right to never let him be alone with your children.