Train + KD Boat on Rhine in 2 days enough ?

For 8 year old boys I would definitely recommend Rheinfels, in St. Goar. It's a ruins, not an intact castle, but it's the only place where they could run free. Other castles in the area, like the Marksburg, in Braubach, are only see-able in a tour, so they wouldn't be able to explore on their own. Of the others that are open to the public, Stahleck and Schönburg are a hostel and hotel, respectively, on the inside, so not much like a castle and not conducive to exploration. I don't know what you want to do in 2 days. The trip by boat from Rüdesheim to St. Goar takes 1hr 40 min, so that will chew up much of your morning. I imagine Rheinfels could take most of the afternoon. From there you could take the train to Bacharach, look around, have dinner, and then go by train to Bingen Stadt Bahnhof and take the ferry across the Rhein to Rüdesheim. Note that there are two Bahnhöfe (stations) in Bingen. The Bingen Hbf is a place to change trains, but not very accessible to the surround area. Bingen Stadtbahnof is on the river, a short walk to the ferry that goes across the river to Rüdesheim. For the other day, you could take the train on that side of the river (right bank) down to Braubach and see the Marksburg, the only castle on the Rhein which was never destroyed.

Sorry, we are staying in Rhudesheim.

Day 1: If you arrive in R'heim at 4 pm, it's too late to do any castle tours or cruises on that day. If you arrived a bit earlier, you could leave bags at your hotel and catch the 4:15 K-D cruise boat north to St. Goarshausen, across the river from St. Goar (arrive 18:05) and return by train to Rüdesheim on the 18:28 train from St. G'hausen (arrive Rüdesheim 18:52.) That would save you some time on Day two that would otherwise be spend on a cruise. Use the trains to visit Boppard and do the chairlift ride, see Rheinfels in St. Goar, see Marksburg in Braubach; there's no rule that you can't see both castles - Marksburg is furnished as it might have been in centuries past and gives you a real look at history in a way Rheinfels (which is also quite good in its own way) does not. You might want to get a Rheinland-Pfalz ticket (daypass, after 9 am weekdays, anytime on Sat or Sun) for Day 2 train travels - it's 25€/2 adults/day, and kids ride with you for free: http://www.vrminfo.de/en/tickets-and-fares/ticket-offers/rheinland-pfalz-ticket/ Buy the R-P ticket at a ticket machine in Rüdesheim or any other station in Germany.

We have done trips on the Rhine from Wiesbaden (mainz?) to rudesheim and also wiesbaden to St goar. the time on the boat did not appear excessive to me and we enjoyed the scenery. We then trained back to Wiesbaden. After some local sightseeing. (as i recall the rudesheim was only a few hours, we got there in time for lunch.