If your ex-spouse has sole custody, does that mean that you cannot have access to the children?

No. Unless there are significant reasons to prohibit it, you will still have access to the children. You can still see them and go to their extracurricular activities. Usually, you can also have access to all their records—medical, psychological, educational, legal. Sole custody just means you do not have a role in making decisions, but you still have the right to be informed about the decisions that your ex makes.