Why is my child's investment income of $4,800 triggering over $6,000 in tax due?

My depending child has $4,800 in investment income this year. We opted to not include her income in our return and instead to file a separate return for her. After entering all of the info, Turbotax is calculating that she owes over $400 in federal tax and over $5,800 in CA tax. Huh???? I don't understand this at all.

 

If we had included her income in our return, it would have added about $600 in tax due.

 

Thanks for any help.