I made a similar item, it was a giant pencil that got marked as the child grew.
Paint it with latex or acrylic paint, (matte finish)
Pencil in the lines.
Use a paint-pen to draw the lines
topcoat with waterborne poly.
http://www.michaels.com/painting/paint-pens/845162977They are getting harder to find because of all the computer graphics that are available now, but rub-on transfer letters, numbers and tapes are still available:
http://www.letraset.com/shopcontent.asp?...-transfersAlternatively go to the auto parts store and buy pin striping tape in two thicknesses. Apply the tape and bury it under several coats of clear.
The only marker that will work in the clear is a permanent marker. Everything else will fail.
The matte finish base coat will take pencil markings well. The clear will protect it from dirt.
Here is my pencil. Only the "2" is painted in and topcoated. All the heights are added by my niece with a permanent marker.
Make sure that they mark the mounting hardware height on the back of the project so that when they relocate it to another location it will retain its accuracy.
The pencil is 6 feet tall, and the three facets allow three children's heights.