Welcome to the forum, Randy from Rocklin.
Frankfurt to St. Goar (Frankfurt > Mainz + Mainz > St. Goar
St. Goar has great river scenery and is a good choice for train travelers and river cruisers. Good choice. Rheinfels Castle is right in town as well.
We do plan to do a KD boat cruise, but not on the transit day We'll be doing it on our full day in the region.
It's a little tricky to advise you without knowing all the time details of your first few days, but I'm going to strong-arm you to do as Sam says anyway. If you do the cruise - the best cruise anyway - on your "full day", then you will need to backtrack to Bingen (which you passed through the day before) by train, then cruise back to St. Goar (yes, the best segment) and you will burn 2.5 - 3 hours of this day doing that. And besides the time savings and train ticket cost savings, why "spoil" your virgin journey to the Rhine and a later cruise by taking the train, when you could instead enjoy the passing castles, towns, cliffsides and vineyards from the best perspective - from the middle of the river? There are afternoon cruises to St. Goar at 14:30 and 16:30 in case you aren't leaving Frankfurt in the morning.
The quickest and cheapest way to get from Frankfurt to Bingen Rhein Stadt station (closest to the boat dock) is by regional train (RMV ticket, kids are not free.) You'll pay €34.50 for a group day ticket from a station ticket machine (which is cheaper than adults + kids tickets.) Show these tickets at the KD dock for a cruise discount.
Then on your "full" day you can truly have a full day. Visit Rheinfels Castle (if you didn't the day before.) Visit another town or two nearby (10 minutes to Boppard and take the chairlift ride and to an awesome lookout? 5 minutes to Oberwesel? 10 minutes to Bacharach? Braubach - home of the truly medieval and absolutely intact Marksburg Castle on the opposite river bank - (a little further - use the St. Goar Ferry crossing + the train.) A full day on the river to these places will run you €23.60 for the whole family for a VRM mini-group ticket (which includes that ferry crossing as well.) Buy that at the station in St. Goar too. (Pssst... it covers the ferry crossing as well.)
You won't have time for everything, but you will have time for lots more than if you cruise on this day as well.
Actually that VRM mini-group pass covers all these places but Bacharach - but you could pay an additional €2.70 each adult (kids free) for a DB ticket that extends your trip from Oberwesel to Bacharach.)