Thank you for the replies. Looks like I've got a few things to try...

Riding herd on budding paleontologists is an interesting business. Not all are young - our very best excavator did her last season, most likely, at age 92. It was quite a milestone for me when she allowed me to sharpen her tools.... At the other end of the scale, a young lady of 16 my first season is now working on her doctorate - how quickly they grow!