Endgame training for a 7 year old kid My 7 year old son plays competitive chess. He can checkmate with king and rook nicely. He can make a queen from a single pawn &amp; king. Now, how to proceed for further endgame training?
When he is mating, is he just following a method blindly or does he understand it?

For instance in the two examples you mention, zugzwang and opposition are important themes, which are also relevant to many endgames. You can find many easy pawn only endgames with these motifs and since they are so fundamental, I'd start with these motifs. Somewhat related, triangulation, key/corresponding squares could also go here. All of these you can do in pawn endgames which, because of the few options you have, are generally easier to understand than if you add any other pieces.

Some more specific topics could be king and rook-pawn vs king (draw/stalemate), mating with two bishops (which is not all that difficult if you understand that two bishops can build something like a "wall" that the enemy king cannot cross and your own king can relatively easily push the enemy king to the border), square rule to quickly assess whether a king can catch a pawn before promotion.

If you think that these are too complicated at his current level, I'd forget about endgames for the time being and focus on opening/middlegame play.