MakeWorkPay vs Payroll Tax Reduction
Look into the effects of the Make Work Pay credit. This credit was figured into the amounts for withholding for 2010. I believe the idea was that the $400 would be split up every week and not just refunded at the end of the year.
In 2011, they removed this credit, but reduced the Employee portion of Social Security by 2%. However it works out, there is an amount around 20,000 annual pay where 2011 has lower withholding if you would make more than 20K. If the 30 hrs at 10/hr was for an entire year, the annual pay would be 15,600. This situation would actually see a slight increase in withholding and therefore a slight decrease in their paycheck.
Here is an article that gives some information on the tax changes for 2011.