"Mettre à la portée de qqn" means to modify a situation or thing in such a way that some of its difficulties which are nearly or quite insurmountable for that person (qqn) are done away with; in doing so the situation or thing is made manageable by the given person.

It can be used in the alternative ways you list.

• Il a mis ce livre à la portée des étudiants de première année en excluant habilement les parties difficiles.

• Il se met à leur portée en n’exigeant pas tout le vocabulaire technique qui est nécessaire et en leur permettant de s’exprimer un peu en langage de tous les jours.

"Mettre qqn-1 à la porté de qqn-2" is rarely used, except if, through metonymy, qqn-1 means the œuvre of which that person is the author.

• On ne peut pas mettre Emmanuel Kant à la portée d’élèves qui n’ont pas été sérieusement introduits à la philosophie, et c’est le cas d’ailleurs pour beaucoup de philosophes.

I would put myself within his reach.

That is, make myself accessible to him. In the context of the book, the adults are incapable of understanding the narrator (or his inner child?), so he has to adapt himself, bring himself down to their level.

Yes, it can be used non-reflexively. You can put anything within anyone’s reach using the pronouns you suggested and so on. Also, here it’s used figuratively, but it can also be literal.

The opposite, frequently seen on bottles of pills and whatnot, looks like this:

Garder hors de la portée des enfants