We must have been lucky then.
Back in 2004 we travelled from The Netherlands/Belgium to Peru: three children, Peruvian/Dutch nationality, one daughter, Dutch nationality, all of them minor of age.
My wife has the authority of all her three children (although I don't recall that we took those documents with us), but I did not have any written consent from my ex-wife.
However no-one ever asked anything, so maybe it was different in those days, or we were just lucky.
Will will however travel on Sept. 7th to Peru. Our daughter (originally Peruvian) will accompany us. When leaving The Netherlands, she is still under eighteen, she will become eighteen in Peru, just a couple of days before returning to The Netherlands (she wants to go back to The Netherlands, rather than building her future in Peru
I hope that leaving The Netherlands will be no problem?
Returning on her own to The Netherlands certainly not, I expect, as she is eighteen then.....