Movement of minors can be a sensitive issue and it's better to over-prepare and over-think where ever possible.
The over-arching priority in bringing minors into the EU is the rights of the child. You should familiarize yourself with its provisions. Member states have adopted local statues along these guidelines.
The grandmother should bring something to show that they are biologically related and both parents consent to the children's travel. Regulations also require that the children be cared for adequately and the grandmother should bring evidence of this.
If possible, bring something that shows the children will not miss school or otherwise become truant. You can also advise the American Citizen Services Host Warden in Poland about the details of the trip.
Related background info: protection of minors.