The airlines say no documentation is needed for minor passengers. Our children are 5 & 8 and I carry there state issued ids for security and peace of mind - just in case. We live in Ohio and you can get a state id for about $8. That solves any issues.
Also, make sure you have a notarized power of attorney for the children. The POA should stipulate that while they are in your care that you are allowed to obtain medical care for them, just in the case they would get sick or injured. DH and I always have a POA for when we both leave our children in their grandparents care for trips and for when we have taken other children on vacation with us.