Okay, made a bit of progress.
Despite the veils between the realms thinning I was unable to obtain a child on which to experiment
As a result I had to conduct a test on myself. There were some strange looks, but no comments, as I padded around barefoot with a buzzing multimeter in my hand and a mercury switch stuck to my heel with a big blob of blu tack.
The results were reasonably encouraging. Unfortunately it was not possible to walk on tip toe without generating transients as I had hoped, however it seemed as if I could obtain two distinct and simple signatures.
For normal walking the multimeter buzzed more or less continuously (i.e. tilt switch contact made) with brief gaps when my foot impacted on the ground.
For walking on toes the inverse appeared to happen. The multimeter was silent more or less continuously (i.e. tilt switch contact broken) with brief buzzes when my foot impacted on the ground.
The mercury switch I managed to get hold of is sensitive in 2D so the 'ball' type switch I have ordered (currently out of stock) may in fact be better.
This was a very quick test and I am not sure how clean the two signatures are but I think it should be possible to tell them apart in software. The results for a child will be different. A child's smaller feet and shorter legs will move with a different rhythm and their lesser weight will result in less jarring on foot impact.
The system will be quite sensitive to getting the angle of the switch correct I think. More experimentation required.