At 18 without parental permission.
Im not sure if they have to have parental consent or not...
Yes you do.
Yes, until/unless the child is adopted.
If she is not the one with custody, yes.
Depends widely on circumstances. Are the parents married and is there any court orders?
Yes. This is called custodial interference, and has the same impact as kidnapping, but with mitigating factors.
Only with parental permission or be being emancipated by the court.
It was pretty stupid of you to kidnap your children in the first place. Why don't you do the right thing?
Yes they can.
He has to pay regardless of where the nother takes the child as a result of parental kidnapping.
No, you can't, especially if you have joint custody, that will be considered parental kidnapping.