Absolutely, to teach something doesn't only test your mastery of a subject, it helps reinforce it.

I've noticed that in the case of the older child, it reinforces their identity as "someone who knows about X", which feeds back into their own further study of X. It also subtly evokes responsible behavior from the older child simply by putting them in a responsible position, and harnesses the younger child's tendency to look up to older kids in service of their learning. Seems like the kind of thing we ought to do more of.