Hi mate, I've been in Canberra for about 10 years now. The 7 year old would love Questacon. The war memorial as stated is fantastic, and even the 7 year old will enjoy the video shows inside at the back of it. It is probably a half day. Mount Ainslie gives a good perspective of the layout of Canberra. If the younger is sports minded, then the Institute of Sport will also be a hit at Bruce ( a northern suburb, 8 ks from Civic).
Parliament house is also good, and a drive around the Diplomatic area where the Embassies are is interesting to see.
The Bridabellas are good at the moment.
2 .5 hrs up the road to Mt Selwyn or Thredbo is good for the snow. You can hire a toboggan from canberra or Cooma, walk up the hills and toboggan down free, or go to Selwyns, hire a toboggan for 5 plus the lift tickets.
It is chilly here at the moment, minus 2 overnight to 13 during the day.
Enjoy.