Visit Nemo, which is a Science Museum, very close to the central station. Canal book trip also the older ones may appreciate the Van Gogh museum. I went a few years ago but it has just be refurbished so no doubt will be even better. Anne Frank house is another one to visit.
my dutch friend just said def do Nemo, amazing interacitve science museum, do anne frank, and dont do rip off canal trip, jsut stay in posh hotel on canal instead and soak up the vibe of canal dont chug down one.. thats us sorted then..
Please do Van Gogh. It is great. Might be worth getting the 8yo to research the pictures and produce a spot the picture sheet for the younger one.
Very near it is the brand new central library which has great views and a fantastic restaurant at the top.
Pedalos on the canals are more fun than a boat trip.
If you do a boat trip (which you might use an Amsterdam card for) then consider using it to get to the Zoo.
Anne Frank is bookable online and their is an excellent BBC Kids serial DVD which helps explain it. My kids knew more about what they were looking at when they went than most adults because of that DVD.