A long time ago, when my kids looked like this:
Here are some tips:
--paint the canvas to match what you want - - last round, I painted it to match the wall color. This time, I painted them yellow to match furniture in the same space.
--give the kids only paint choices you want to use. Above, the room was green so I gave them black and silver and pink (there were pink accents in flowers in that room). This year we have a red couch and I pulled some other colors out of fabric I want to incorporate into the space.
--Paint somewhere that can get messy - we're in the garage. Side note - last time, a couple of years ago, I left my van in the garage - mistake! I dropped a thing of paint, so while getting stuff to clean it up the baby at the time was washing himself in the paint and smearing it onto my hubcaps.
--Wear things that can get ruined - pjs was our choice (and undies), but anything with do.
--I always add their names and year. They want to do it, but I want it to be legible and crisp, so I tell them that's a grown-up job.
--I had a whole talk on how you can paint without having to paint "something" - you know, trying to get them to go abstract instead of a picture of a dog. We discussed patterns, lines, dots, etc. All said they got it. Immediately my daughter started drawing a girl and my preschooler was writing on hers - whatever, it's their paintings (that's the hardest part! let them do it all!)
--Take pictures of the process - it's fun for them to look back on the paintings taking shape.
--The last time we did this was about a year and a half ago - - here's the post I wrote about it on my family blog....