The Bakfiets bikes come with two straps for the children’s bench. Unfortunately the buckles on my second hand bike had worn out and broken which meant they were completely useless. The buckles are sewn into the straps and you can’t get replacement straps for the Bakfiets. Luckily there are fantastic side removable buckles made by a company called Sea To Summit. They can be used to replace a broken buckle without restitching the strap.
You can order the replacement buckles from Amazon here.
I hope this helps somebody.