Traveling from Krakow to Vienna in June (group is three children 7, 5 and 3 and 3 adults) was thinking about using the train but has anyone have any recommendations on possible private car hire transfers on this route? Or any comments on the possible side trips/stops that are available?
Fly Krakow to Budapest. 4 days later train to Vienna. Cost maybe $150 each.
Bus will be cheapest but take the longest, train will cost a bit more but take less time than bus. Quickest way, and probably the easiest with that many people, would be a private transfer. Although it will cost quite a bit more it may be worth it. I don't have personal experience with either but you could try: my day trip or viator.
Train service looks much improved this summer, with a direct afternoon train taking only 6 hours between Krakow and Vienna starting April 13 (faster than trains to Prague or Budapest). Morning departures require a quick change of trains en-route.
With six people I would sure look at a private transfer. Budapest to Krakow runs 520 euro for a van, so I suspect no more than that to Vienna. Divided by 6, and its a bargain. This is the Budapest service so you can see the sort of thing you are looking for: https://transferbudapesthungary.com/price-list.html
For 6 people the best will be to rent a van with a driver, if you share total cost it will be less than 100 euro per person. Here is reliable transfer company who can organize your transportation less than 500 euro https://www.besttrips.tours/trips/product/to-krakow/budapest-to-krakow/