It's not very challenging at all. A good while before
the activity requiring the supplies you send home a note that
says the class is collecting pizza boxes for a project and
asking folks to send in whatever pizza boxes they have. When
the required number have been collected, send another note home
saying mission accomplished. Problem solved. You just have to
plan ahead.

Good in theory, but would you honestly get your donation there as
willingly and as quickly if you knew it was just going towards a general
total, rather than having the pressure of possibly on that day if you
didn't send the child with the relevant thing that they would be upset
at being left out - if I'm the only parent that would in practice end up
behaving like that then your system would work, I may well have a unique
personality trait, but I doubt it.

I'm not the parent who's jumping up volunteering all the time, but in my
head I am very much thinking for the good of the group, so in theory
it's a great idea and the first time it was done, I'd probably be right
there with my contribution, but the next time, once you'd removed the
guilt feeling of your kid being upset, I might not go into the garage in
the rain to get whatever was needed, or take a detour to pick it up or
whatever, because when it comes to the crunch, I'll be honest and admit,
that it's the fear of having to deal with a disappointed child because
they couldn't make a train like everyone else that drives getting hold
of that big box, not that making a train is some great learning
experience for the group as a whole.

If I really am the only parent that is like that then great, the
relationship that I describe isn't challenging as it can be acheived by
simple passing of notes, but to me, passing notes isn't acheiving the
relationship that provides the understanding I suggested, it's something
more that means when I screw up and forget that train day needed a box
AND a soft toy, that the teacher was able to be creative and make the
kid not even notice that mummy messed up, rather than being frustrated
that thing weren't going as planned.

Cheers
Anne