Is it legal to take kids out of state without the other parent's consent, if they are with the parent that does not have cusdoy?
If they had rightful possession of the kids and a court order did not prevent them from going out-of-state without notice to the other parent, then yes-- they could legally take the kids out-of-state. The main thing is the court orders. Many custodial parents get upset when the non-custodial parent travels with the kids, but as long as there are no court ordered restrictions they are free to take their children on fun trips, vacation trips, or business trips-- as long as the return the children when their period of possession ends.
If the non-custodial parent takes the children out of state when it is not their turn to have the children, then this would not be legal-- because it interfered with the custodial parent's period of possession.
If the custodial parent has issues with the travel plans made by the non-custodial parent, then they can file a motion with the court and try to have restrictions placed on the non-custodial parent. Keep in mind, however, that judges will sometimes impose restrictions on both parents, even though only one parent is pushing for travel restrictions or consent for travel. This means that a custodial parent could end up having to include the non-custodial parent more in their lives than originally hoped or planned.