Alone, it is probably best to have this document-
but I can tell you from personal experience that, if travelling with adults, they dont seem to enforce this very much.
I have brought, and taken, my under 18 year old kids with only one parent, and I have brought in under 18 unrelated friends of my kids- so, while they were travelling with adults, they did not have EITHER parent with them.
Migraciones just stamped her passport, and didnt say a thing, either coming or going. No additional documents were asked for.
These were all kids with US passports.