What is the procedure to transfer a percentage of shares of a Pennsylvania S corp to my two children? Scenario as follows:

Owner - 100% of shares
Transfer 25% of company to child 1
Transfer 25% of company to child 2
I will own 50% of the company

Who do I need to talk to so I can transfer 50% of the company? Does my accountant need to do this? I'm willing to just gift these shares to them. Is there a form that needs to be supplied to the state or does the K-1 for our tax returns now reflect this?

The articles of incorporation state the following:
250,000 shares of common stock - $0.10 par value.
1,000 share of preferred stock - $100 par value.
5 1/2% dividends payable out of the profits of the corp when earned, non-cumulative, and non voting.