But if you are taking children that are not your own these are the following things that I would bring.
Birth Certificates
State ID
A statement from the parents staying you have the right to take their children out of state.
And for good measure, I would get copies of health insurance cards and a letter that stays you have the right to have the children treated in emergency.
To tell you the truth, I don't think you will be asked for any of these things at the airport but they still would be a good thing to take with you.