Next August I'm intending to travel by boat from Wien to Budapest, Is that a good choice ?
We are a family of 4 (couple and two children 12 and 17 years),

It's five and a half hours sitting in a hydrofoil along one of the less scenic stretches of the river. The journey is three hours by train.
If you want to see some scenery on the Danube, do a trip in the Wachau region (www.ddsg-blue-danube.at); the ships have open decks and restaurants.

Is the Wachau region nera Wien ?
How to reach it ?

There is a map of the Wachau stretch of the Danube on DDSG's web site. Melk and Krems can be reached by train from Wien in an hour or so (www.oebb.at).

I would consider it a bad choice. We investigated the same thing some years ago when we traveled to Prague, Vienna and Budapest. I posted here with the same question and received only negative responses. Long trip, not scenic, expensive and at that time smoking was allowed in the enclosed cabin. That did it for us. We trained from Prague to Vienna and then from Vienna to Budapest. These trips were very pleasant, both trains had dining cars and I'm sure we were much more comfortable than we would have been on the hydrofoil.

We took a local train to Szentendre from Budapest and the hydrofoil on the Danube back. Perfect day trip. Also did a night boat ride on the Danube, which was perfectly beautiful, with all the lights on the Chain Bridge, Parliament Buildings, etc. Very pleasant sailing along, drinking complimentary champagne.

Budapest is a wonderful city. Enjoy your trip.

Where to buy that kind of trip (local train to Szentendre from Budapest and the hydrofoil on the Danube back. Perfect day trip. Also did a night boat ride on the Danube)
Did you buy it at the hotel or ...?

Schedules and fares for the trips from Budapest are on www.mahartpassnave.hu. There are also pictures of the hydrofoils and their seating.

Pamane: These were just things we did on our own, no need to "buy a trip." We stayed on the Buda side and had a metro station (I'm sorry I don't recall the name) walking distance from our hotel. We caught the train to Szentendre there and took a cab from the train station (end of the line) into the center of town. When we were ready to return, we walked quite a distance to catch the hydrofoil, bought our tickets and boarded.

Our evening Danube cruise again was easy to arrange. I think our hotel made a reservation for us, and we took a cab to the Pest side to board. The dock was near the Marriott Hotel. We purchased our tickets and boarded. You can watch the narration on small TV sets thtoughout the boat, and listen to it in English (or many other languages). We were provided with earphones and just plugged in for English. Both hard and soft drinks were complimentary. We sat toward the rear of the boat in an outside area.