If I accidentally selected the "Child's Income (Under Age 24)" option under "Less Common Income" heading, how do I remove it?

You should be fine as you have not entered any information.  It should not create the Form 8615, Tax for Children Who Have Unearned Income.  To make sure, you can check and remove the form from the program if it creates one.  Here are the steps:

In TurboTax online,

• Sign into your account, select Pick up where you left off
• From the left menu, select Tax Tools, click on the drop down arrow
• Select Tools
• Under Other helpful links, choose Delete a form
• Select Delete next to Form 8615 and Continue with My Return