I have twice recently been asked what to do in Amsterdam with kids... Clearly need to blog this but for now.
I recall you are biking folks, so rent some bikes, you can get one with a variety of child attachments for the 4 year old. If after walking through the city that kind of biking doesn't look fun... Rent bikes and visit Vondelpark, regardless of bike riding Vondelpark is awesome to walk through.
I frequently stayed very close to Anne Frank's house while there as my old office was walking distance from that.
- If you do a boat tour I highly recommend "Those damn boat guys" We have done that twice with Theo (who is now 2)
- they are smaller boats/groups, and less stodgy very funny and factual kind of stand-up comedy like history