How long is too long for driving with your kids? How old are your kids?
Lucerne is 3 hours from Fussen and on the way to Murren. Depending on the age of your kids, there is a fun little zoo in Goldau outside of Lucerne with deer who wander freely around the grounds that you can feed as you walk. It would add 30 minutes onto your drive but if you stopped there for a couple of hours it would break up the drive for the kids and allow you to get to Murren in one day. If your kids are older, you could plan to do one of the
Between Murren and Trier, Colmar and/or Riquewhir would make good stops. Riquewhir is traffic calmed so your kids could have some freedom to run and explore without running into traffic. It would make a lovely stop for a night, as would Colmar. There are a lot of other great little towns to stay at in this area as well. If you wanted to just break up a long day trip, Riquewhir (or any other small town in this area) would be a great stop. Haute Konigsburg would be fun for older kids, as mentioned above.