‘I am having my 18 month old granddaughter visit me in San Diego from South Dakota for 3 weeks and her mother and father were never married nor do they have court visitation set up. Can the father stop my daughter from allowing my granddaughter to come stay with me for 3 weeks?’
If there has been no legal action at all, then each parent has as much right as the other to travel with the child. If there *are* any court orders, then your daughter would need to carefully review the order to see what it says about traveling outside of the state. Most, if not all, states have standard orders that are included in each family law action that prohibit removing the child from the state until the court resolves any issues related to custody and parenting time.
Should your granddaughter’s father file an action with the court prior to the trip, the odds are good that the filing of the action would allow him to keep your granddaughter from leaving the state until there was a court order outlining each parent’s respective parenting time schedule.