What a fun trip that wil be! Hope you'll have a few extra days for sightseeing on both ends, so much to see!
Clothing like those Pearl Izumi Impact capris will be perfect. You won't see much Lycra or high-viz being worn in Belgium and the Netherlands. Even for trips like this many people just bike in street clothes. Stores like REI carry a lot of casual cycling clothes these days - not Lycra but looking more like regular street clothes with a comfortable cut designed for casual cycling. Sometimes with chamois. That kind of stuff will blend right in and be comfy to ride in. Don't forget to pack some rain gear.