If I were u,

I would call her or talk to her to say, "I was not able to send your kid back until the whole party was over. I wished you could come and pick up you kid earlier. Now I am very upset seeing your hu*****and being upset like that. I just want to explain the whole situation to you, and hope you would understand."

No apologize! Because it's not your fault. Instead they should apologize to you being rude and inconsiderate to you.