I used to volunteer teaching after-school middle school classes with an organization called Citizen Schools and one big part of their methodology was trying to get kids to demonstrate their knowledge and pass it on to others in the end of whatever lesson they were doing.
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.
Or you'd better do a lottery to assign children 1-on-1.
Further experiments are needed.
The reason I found this interesting is that it gives something to do with class size disparity: have the "best" students in the oversized class work in an independent study format for that hour. It's so simple.