Over many years I took my kids by train to see almost every country in europe (all except Moldova, Kosovo, and islands...). Mostly we used inter-rail tickets as kids up to 12 are free. Now they got older it's more difficult, but a few weeks ago took my family from Wallonie to Catalonia - for 29€ each person, all the way from belgian border to spanish border (with some hours in Paris). Can also get good prices from DB crossing three countries (e.g. Belgium - Italy, or Poland ). It helps to know the routes (use openrailwaymap) and experiment with the options (add 'via', change stopover time etc.). Indeed it’s frustrating that every country system is different.