Many successful sport horses and approved stallions were trained at the Houwen family at a young age. A couple of successful references are:

KWPN approved stallion, won the Stal Hendrix competition as a four-years-old and ended up second in the Dutch championships under the saddle of Liza. Sold as a five-years-old to the United States.Impala SMH (Zambesi x Cardento)

Impala SMH (Zambesi x Cardento)

Trained up to international 1.45m level by Liza. They have won the 1*-GP of De Peelbergen. Via Tal Milstein he is sold to Italy, where he is now performing at 1.40m level with Matteo Orlandi.
C’Est La Vie (Namelus R x Nimmerdor)

Fieke Houwen won the Limburg championship at Z2 / 3rd-4th level. He is now performing successfully with Bo Oudhof in U25 Grand Prix level and they competed in the EC for Young Riders
Colt Sollenburg (Vivaldi x Osmium)

This elite mare participated in the final of the Dutch championship for four-years-old with Fieke, and was sold to Sweden afterwards.
Gardenia Sollenburg (Vivaldi x Falco)

The striking Warnike was sold via Stephex Stables to Eric Lamaze. She first foaled two offspring and came under the saddle of Kristian as a six-years-old. They made great progress and won the silver medal at the Dutch indoor championships on 1.20m level. 
Warnike (Heartbreaker x G.Ramiro Z)

Trained up to 1.35m by Liza. Sold to Catherine Pasmore and is now competing under McLain Ward at international 1.40m level.
Spring Key SR Z (Spartacus x Calato)

Achieved great results in the young horse competitions with Liza, such as winning the GMB competition for four-years-olds. Approved for the NRPS Studbook and sold to the United States.
Gabriel SMH (Azteca VDL x Burggraaf)

This outstanding jumping talent was very successful as a youngster with Kristian. He was approved for the NRPS and sold to Mexico. Under the name Parrandero, he is now performing at international 1.50m level with Enrique Gonzalez.
Classe HS/Parrandero (Clarimo x Cassini I)

After striking results in the young horse classes with Fieke Houwen, Spielbergs Cartiër has developed into a good Small Tour dressage horse. He is sold to Carolien Pauwel.
Spielbergs Cartiër (Spielberg x Jazz)