If you were paying the AMT, it would show up on Form 6251, line 11. It would also show up on Schedule 2, Line 1.
As you have discovered, the increase in tax is not solely due to the capital gains on your stock sales. Also by having a larger gross income, it has begun to phase out or eliminate your child tax credit. This may be the case with other credits as well as many of them do have an amount where they are phased out or eliminated.
