As with many children my grand kids love to play with pens, crayons etc. so an induction of a ruler initial to aid with straight lines seemed appropriate with the view of assisting with numbers, counting and eventually measuring.
Making a full size 30cm ruler for them was relatively easy, but it soon became apparent that there are some drawbacks with wooden rulers.
The main problems is that they do have a tenancy to warp/bend as they are thin and long.
Another problem is that for young children like my grand children they are a great length for using as swords for fighting or hitting people and they can also splinter and break.
I have not given up on the idea of making wooden rulers for kids, but I am going to redesign a shorter 15cm version that should get over some of the problems listed above and I am also going to experiment with some different materials or pre manufactured rulers.