We can fix the Medicare discrepancy by creating payroll liability adjustment. This will help adjust the tax liabilities for your employees to have a correct Year-to-Date payroll details.
Please check out this article for the steps: Adjust Payroll Liabilities.
With regards to filing W-2 form for your daughter who is not yet in the legal age, it'd be best to ask some pieces of advice from your accountant. Each state has different filing requirements. Therefore, she may be required to file in your resident state.
For your medicare deductions on your 941, you can try to run the Payroll Detail report to see if there's a transaction that's causing the discrepancy.
Please let us know if you have any questions. Thanks!