Skip, have you seen the posting of using Command brand strips for picture hanging? They work great and are easily removed or re-positioned for different seam allowances. I have used them even when my 4 and 6 yr old GDs are sewing. Actually, one of the 4 yr old twins uses the foot petal on the table with her hand, and I am the one guiding the fabric through the machine. She did a great job of stopping just before the pins in sewing her sundress. She will be sewing soon, I'm thinking, as she is very interested in the sewing machine like her older sister. I've just got to figure out how to put the finger guard on my machine first.
When using any type of devise to mark the seam allowance on the machine, when I need to use pins, I put the pins in from the LEFT side and pull them out with my left hand. That way the seam allowance devise, what ever it is, doesn't interfere with the pins.