To get to Prague from Vienna is pretty easy, there are again '''RailJet''' connections every two hours. The trip in those fast trains takes about 4 hours. If you get going early (around 7:00AM), you're in Prague before lunch. The best connection is probably the one at 9:00AM, which gets you to Prague at 1:00PM, at which time your hotel room there should be ready for you.
There is food on the train, and it's not even bad. Since it's a long ride, you may wanna consider 1st class tickets to get a bit more room. At a minimum, you should make seat reservations, to ensure that you sit together. The train has plugs to charge your devices, and has also free Wifi.