If you take a measuring tape and paint the non-numbered side, you can both keep track of height and have a cute keepsake to give them when they're older. :)

E: I realized what community this was after making my suggestion. Ah, well. It's still a tool you might use in woodworking. 😅

This is what I did for you our kids. Used a bull nose but in a router for the engraving. The graduations were done with a straightedge while the numbers were freehanded over pencil lines.

I then spray painted it black and sanded off the paint from the flat surfaces, leaving it paint in the engraving troughs.

Their heights are simply penciled on.

We're pretty pleased with it. It looks nice and doesn't feel out of place.

I hung up a maple 1x6.

The left side has an attached ruler with both metric and ft/in.

The right side has large vinyl numbers for feet (mainly just to look nice).

In between the heights are marked with multicolored Sharpies indicating who is measured and the month/year.

I built a height tracking tree - can't remember the size of the pine plank, but simply carved out the edges with a jigsaw and sanded it all. Used the cut off pieces to extend one or two of the branches horizontally

I put nubs of branches all the way up the sides. I know trees don't have branches that low on the trunk, but it gives them something to hang stuffies on.