Be honest, six days for Hokkaido are rather short.
I basically agree with the plan by TMC3012, but one day for Asahiyama Zoo may be too rush... This zoo is very nice for children (if they both like animals, in particular penguins). If so, maybe you have to think about dropping Otaru (or one day of Sapporo, as your first and last days will also be in Sapporo) from your plan.
Any possibility to extend one day more? Pls try, and your trip can then cover Sapporo, Otaru and Asahiyama Zoo in Asahikawa.