I am Dutch, and although I have no plans to ever return to my home country for various (other) reasons, I think Amsterdam is really easy to get around with. One of my other favorites is Barcelona, not too difficult. Italy definitely less with small children. Scandinavia is a good option too, though very expensive and it may be quite chilly. Germany has a lot of nice places (Munich, etc.) and so does Switzerland, well organized, etc. My parents live in Belgium, Antwerp is nice and easy with kids and so is Brussels. Brussels is a 2 hour train ride from Amsterdam, Antwerp a little over an hour. Trains go very frequently.
My vote would be Amsterdam and Antwerp / Brussels. Don't count on great weather ... you never know ...