You can if you have joint custody. If your ex has Sole Custody this means that you cannot make any decisions as to where they live or go to school. Your ex can determine when the kids see you and for how long. Just keep trying to be a part of their lives.
Sole custody means that the parent with sole custody makes all major decisions, they still have to let them see the kids on the days of the visitation arrangment. to leave the state you must have written permission.
"Sole Custody" means that one parent has similar rights and responsibilities as a single parent. You can only visit or take your children with the consent of the custodial parent.
Sole custody means the person that has sole custody of the kids, they can say weather you can see them, or if you can take the kids anywhere. That person has most of the say about the kids going anywhere.