Anybody else had a problem with PA state underpayment penalty? Turbo Tax calculated too much for my mom and too little for my son. Never had an issue before.

You can either choose to have Pennsylvania figure any estimated tax penalty for you, or you can complete the Underpayment Penalty questionnaire in the state section of TurboTax when it comes up. You don't have to complete the Form 1630.

If you later receive a notice about an underpayment penalty from PA, you will have the opportunity to respond if you don't agree with their calculation.

Each state has its own laws regarding estimated taxes. In Pennsylvania, estimated underpayment penalty (interest) is imposed on taxpayers who fail to make estimated payments or fail to make timely estimated payments. Estimated underpayment penalty is calculated on PA forms REV-1630, Underpayment of Estimated Tax by Individuals, REV-1630A, Underpayment of Estimated Tax by Individual Farmers, and REV-1630F, Underpayment of Estimated Tax by Fiduciaries. 

Estimated underpayment penalty is calculated on the amount of underpayment for each installment period multiplied by both the daily interest rate for the tax year (and the succeeding tax year if applicable) and the number days unpaid or paid late. See here for more information.