What you might be able to find (or might have found and not recognised) is a flatish two pin plug. The pins are of different lengths, made of platic with a brass strip running along the top end of each one. The wires from your transformer/controller go into the back of this plug and then you slide the plug between the metal track and the plasic web that makes up the sleepers and holds everything together.
Because of the age of your set you'll find that your track is very chunky by modern standards and you may have difficulty obtaining the correct connector. take a piece of track to your local model shop and see if they can assist, it may be that modern PECO connectors might fit (but don't quote me). Failing that, do as you're doing now, slide the wires between the metal track and the plastic web and away you go, nothing wrong with that at all, or you could solder the wires onto the underside of the track. Both way's are perfectly proper although the latter may be easier to for a 9 year old to play with (once you've done the soldering).
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