Please write an email to the parent who complained about your child hitting their child.
Hi, David:
Please pass my apology to Simone on Jessie's behalf. I hope he has a swift recovery. And thank you for sharing this with me. 

My wife and I will talk with Jessie about her bad behavior. We will try to find out her reason/motivation and emphasize the rules of interacting with and respecting her friends. 

We have not received a similar report in the past. So, we never knew this as a problem before. But we will take it very seriously and do our best to help her to fixe it. Please let us know if this even happens again. 

Please accept our sincerely apologize.