It's a good thing to have regardless, we haven't travelled to Cuba with under 18's, however we have travelled to Jamaica and the DR with under 18's, and never had a problem.
Here is an excerpt from the Canadian government travel information/advisory site specifically about Cuba:
Any adult travelling with children may be required to show evidence of parental/custodial and/or access rights. Foreign and Canadian authorities may also require evidence that the adult has the consent of the parents, legal guardian, and/or the court to travel with the children. Some countries may not permit children to enter or, in some cases, leave the country without proper documentation such as a letter of consent or a court order.
I would say if in doubt, contact the Cuban Embassy in London and see what they have to say, and if you can, get it in writing via email. However, my thoughts would be to get the permission letter from the parents either way, just so you're not stuck should they decide to start hasseling you.