You may want to check the entry/exit requirements for the specific countries. But usually, a notarized Permission to Travel Letter, signed by both parents will be enough. I also get a letter giving me permission to obtain any medical treatment.
Agree with the above, but also with the airlines. I would just go ahead and do it. I've been asked when I travel with my own kids without their dad, so even in that situation I get one from the father. You will need both parents to have given consent if they want it.
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