What I try to do is improve outcomes for the children of relatives and friends. I want to use readymade teaching materials and the authority of their authors precisely because I won't have much time with those kids and they have no reason to take me seriously.

Unfortunately, the kids don't have much reason to take the authors or their materials seriously either, unless you give them one. Schoolkids generally learn the contents of their study materials (to the extent that they do so at all) because the authority figures in their own lives make them, not because they acknowledge the materials as valuable sources of information.

The more intellectually curious will study materials that they personally find interesting, while the more conscientious will study materials which will impress people they think are worth impressing.