In the Netherlands there are 3 main organisations providing training courses. The VDWS (German based), KNWV (Dutch based) and the IKO (International). All three provide a solid training and you can choose based on your goal of teaching.
The IKO has the largest international use base, so if your intention is to travel and teach kiteboarding around the world, you will find the IKO Instructor certificate will give you the most opportunities.