How do I add another step or filed to the checkout process

I am looking to add an extra field or step to the checkout process. Basically I want a customer buying tickets on the site to enter in whether or not they will be bringing children to the event. This is because children get in for free but advanced notification is needed in order to accommodate any children at the event. So basically in the checkout process, maybe the shipping page add a filed asking the customer to enter number of children and have its result sent in the email to the site along with all the standard information.

Thank you