Showing or hiding a field depending on a value of another field...At the content types there is now a tab: manage dependencies
You add a 'control' field and a 'Target' field. The status or value of the 'Control' fields wil determine if the 'Target' field appears or not.
If you have 1 child, the textfield for the first name will appear.
I add a second field for collecting the second name. The first name field has to be visible too. So i change the settings