I find that doing art with kids is very enlightening to me as an artist.   They can teach you so much.  If you have children of your own you already know what a wonderful quality-time activity art making is.  Here are some ideas that may add to your fun:

• Save everything-  bottle caps, buttons, scraps of wood, house paint, cardboard tubes (paper towel rolls, etc.), foam, fabric...  Kids will find a way to use it on some project.
• Magazines- cut out body parts, food, objects, cars, bright colored spaces, and so on for collages or surface treatments.  Department store shopping bags are often great sources of deeply colored, tough paper.
• Buy Cheap-  Kids don't need professional supplies.  You can buy washable paint by the pint (it's cheaper in bulk), unclaimed house paint (latex- don't use oil based paint with young kids) can be had for a couple of dollars at hardware stores, Chinese bristle brushes are under a dollar and last a long time.
• No Rules-  anything goes (assuming nobody can get hurt).  There's no right way or wrong way to draw, paint, sculpt... no matter what.  That's half the fun.
• Don't ask, "What is it?"-  This implies you expect them to do it differently the next time, or that they've failed in what they're sharing with you.  Enjoy it for what it is (they understand it better than we do anyway).   Instead you might say something like, "Tell me about this" or "I think that's interesting.  What do you think about it?"
• Avoid Oil and enamel based paint, and all pastels-  Solvents are dangerous and the dust from artist pastels are usually toxic (believe it or not,... especially when we tend to blow the dust off the page).
• Be ready to help with any hammering, drilling, sawing, cutting, etc. that they can't do or shouldn't do, so that process doesn't get in the way of the energy.  Let them be the boss with this.

Those of you outside of the U.S. may think the following point trivial, but this is an issue in America:  Numerous academic studies have revealed that regular exposure to art and music studies in schools improves academic performance in so called non-art related subjects.   This is not surprising.  There are few areas in life in which one can be successful without drawing upon both sides of the brain.  Basic problem solving, advanced mathematics, physics, engineering, and so on require creativity and the ability to think abstractly.  Besides the utilitarian benefits, a person has a better chance of being well rounded if their educational experience provides that opportunity.  A child is fortunate if her/his school includes art programs in the curriculum.

In my opinion, of equal import is the inclusion of general studies, technology, and so on in fine art programs.  Many young people who have answered their calling to become artists have enrolled in "Colleges of Art" and "Art Institutes" which have rich and deep studio programs, only to find themselves lost regarding what to paint or why to paint once they leave the college.  This is a disservice to the students who are better served with a well rounded, broadly exposed educational experience.