I've recently started playing X-wing with my kids. They're too young to understand the complex rules of the full game. English not being their native language isn't helping either. We stick to the basic rules from the 2 page starter leaflet from the (red) core set: no upgrades, tokens, critical hits,... only the basic ship characteristics. To the true x-wing veterans this may sound boring, but trust me, with young kids this still is fun.

We’ve got the red core set, but we’re looking to expand our collection. A second core set seems a no brainer. But where do we go to next? What would be good purchases that balance each other out, based only on the ship’s characteristics, and preferably only from the original trilogy?

Sorry if this has been asked before, but I did not find an answer to this specific question.

the millennium falcon and the slave one could be a fun pair of ships.

Agreed, big ships looks cool to kids, and both ships are pretty iconic.