Not sure how old your daughters are but I'll assume you don't want to go to the big Oktoberfest (all the towns will have them so maybe find a small town festival, I've been to a couple and they're super fun and less crazy than the big ones).
On the front end I would consider Strasbourg France, only a couple hours away by train and it's a beautiful little town. Perfect for a couple days and then take the train to Volkach for work.
Personally, I love southern Germany the most. So after work (if it was me) I'd probably take the train down to Fussen, close to the border of Austria. The Neuschwanstein & Hohenschwangau castles are close by and there are other day trips available. Innsbruck Austria is 3 hours away, Seefeld Austria (one of my favorite towns) is right there too. The trains are super easy to navigate and the views are second to none.
I personally don't think the cities in Germany have all that much to offer, especially Frankfurt and Munich. Berlin & Dresden are better but a lot further away. If you share how many days and the types of things you and your family would be interested in we can provide more accurate suggestions.
Feel free to reach out with questions or if you need help planning the trip!
Luxury & Adventure Family Travel Agent
Personal Travel Concierge
Europe • Africa • S. America
www.voce-travel.com
Kyle.regitz@fora.travel