I really liked
Kruistocht in Spijkerbroek (Crusade in Jeans) by Thea Beckman.
It's about a boy from the 20th century who accidentally winds up joining the
Children's Crusade in 1212. Dolf, the main character, was wearing jeans when he got into the time machine, hence the title. He uses his modern knowledge to save a lot of children, which leads him to be accused of witchcraft.
I also liked
Het malle ding van bobbistiek (the funny thing of bobbistics?) by Leonie Kooiker.
Bobby invents a clay-like paste, which he names after himself,
bobbistiek. The paste has all sorts of useful characteristics, which enables Bobby to build the
"Malle Ding" (Funny Thing), which is a helicopter-like vehicle. Then Bobby and his friend (or brother) do fun things with the funny thing ...