... he'll be chomping at the bit to get at the machines, have to keep thing under lock and key whilst he's about, caught him trying to switch the lathe on.

My daughter is now 3 1/2 and about 3 months ago, knowing "dad is in his workshop" wasn`t enough and she wanted to join me. She now has a little seat setup in the corner of the workshop with her pens on it. She is right in the corner and of course I don`t really do very much when she is with me (usually just tidying up!) but she loves asking what everything is and what it does!
However, your comment regarding lock and key is very true! My routine of leaving my workshop has now changes to, turn off lights, turn off power, lock up, check I`ve locked, put the key high up on the shelf, double check I`ve locked.
I can`t wait until she can join in a little more... I`ve already ordered another copy of "build a paper clock" book .... my plan is to take things slowly, one step at a time and learn to use scissors safely first lol
Chris