what happens to 30hrs free childcare if income exceeds £100,000

My understanding is that at the time you reconfirm, you are saying that you do not believe you will earn over £100,000 that tax year. Therefore, if the bonus is truly unexpected then you are fine but need to declare that you're above the threshold at the next three months (unless you're next reconfirmation date is in the next tax year and you think you'll be back under £100,000).