Your itinerary seems reasonable and not very difficult to accomplish, so it's definitely not very aggressive
Praha-Cesky Krumlov: your best option here is purchase in advance group ticket via Czech Railways
eShop, it will allow all 4 to travel Prague-Krumlov with change in C.Budejovica for only 660 CZK (app. 26 EUR)
Cesky Krumlov-Bratislava: here you actually have like 3 options. First one will be to come back via Prague, where to catch train to the border, second will be catch local train C.Budejovice-Brno (
via Jihlava, Okrisky) and then from Brno to the boder. And finally third option will be travel via Austria (C.Velenice-Gmund-Vienna). Second option cost 1059 CZK (app.42 EUR) for ticket C.Krumlov-Breclav (last station before Slovakia)+either international ticket Breclav-Bratislava (28 EUR for all 4) or alternatively just
purchased online ticket Kuty(first Slovak station after Breclav)-Bratislava (13,12 EUR for all 4). While route via Austria is interesting because you can actually get group ticket (
Einfach-Raus-Ticket), which will get all 4 from the Czech-Austrian border (C.Velenice-Gmund) to Austrian-Slovak border (Wolfstahl) for just 28 EUR. While ticket C.Krumlov-C.Velenice cost just 281 CZK (11 EUR) for all 4 +
bus Wolfstahl-Bratislava just 1,5 EUR adult or 0,75 EUR <26 per person, thus just 43,5 EUR in total for all 4 for Krumlov-Bratislava via Austria (vs. at least 55 EUR via Czech Republic)
Bratislava-Budapest: basically two (and a half) options: a) direct international ticket Bratislava-Budapest (17,5 EUR per person) or b) domestic ticket Bratislava-Sturovo (25 EUR for all 4), then cross the bridge over Danube to Hungarian Esztergom (which is a really nice place itself), from where you can catch already domestic Hungarian train to Budapest (1100 HUF per person or app.3,7 EUR). As you can see, second option is just 40 EUR for all 4 + plus chance to see Esztergom (or even Visegrad and Szentendre) on the way.
Half option is rather
cheap bus from Bratislava to Budapest, just in case you don't want any hassle with changes or overpaying for direct international ticket.
Budapest-Venice: actually lately it gotten a bit complicated, since they canceled direct overnight train, thus now there is no direct rail connection. But anyway it's possible to travel not very expenive: a) most fancy and expensive way is to go to Vienna (13 EUR per person) and catch there direct overnight train to Venice (
from 29 EUR when purchased in advance); b) might sound a bit strange, but cheapest solution is just to fly Budapest-Venice(Treviso) with RyanAir; c) catch daytime Budapest-Ljubljana train (35 EUR per person), spend night there and then continue to Venice via either Nova Gorica-Gorizia or Sezana-Triest (roughly another 20 EUR per person)