In general, either parent can take their child wherever they wish within state lines before their divorce has been finalized. If the trip in question is outside the state of Illinois, the parent who wants to go on the trip needs to get the other parent's written consent for this.
A parent generally needs permission from the other parent before travelling with the child especially if the travel will impact the parenting time of the other parent. However, if the separation agreement dictates how travel should occur, both parents are expected to follow the terms of the agreement.
In the USA, though, unless there's a court order requiring permission from both parents, a child may travel (even to overseas) with either parent/guardian without showing proof of permission from the other parent.
A letter of consent, preferably in English and notarized, from the other parent or signed by both custodial parents. The letter should say: "I acknowledge that my child is traveling outside the country with the name of the adult with my permission."
In the USA, though, unless there's a court order requiring permission from both parents, a child may travel (even to overseas) with either parent/guardian without showing proof of permission from the other parent.
If the child is accompanied by only one parent, the parent should have a note from the child's other parent. For example, "I acknowledge that my wife/ husband is traveling out of the country with my son/ daughter. He/She/ has my permission to do so."
Is a minor travel consent form required for domestic travel? No, a minor travel consent form is not required for domestic travel within the U.S. However, it is recommended that parents and/or legal guardians carry a notarized letter of consent when traveling with a child.