Upvote:2
I will use the Canadian recommendations -- when entering into a foreign country they are very unlikely to look up the laws of your particular home country so this is good as a general guideline. A few choice quotes, but you need to read the whole page:
A consent letter is not a legal requirement in Canada, but it can simplify travel for Canadian children, as it may be requested by immigration authorities when entering or leaving a foreign country or by Canadian officials when re-entering Canada
We recommend that you talk to a lawyer about the legal issues that apply to you and your childrenβs unique situation
Most importantly:
We recommend that the letter be signed by every non-accompanying person or organization with the legal right to make major decisions for the child
So while the parents are not involved they have the legal right from what I understand (but you should know this much better) and if so, border guard might ask a consent letter from them. But, as said above: this isn't a legal requirement at least not in Canada.
However, I am just a random stranger on the Internet and I am not a lawyer (I do not even play one on television). I would look into adoption to simplify this situation ongoing.