A few weeks ago I have a fantastic idea that I would get the mat out that we normally put under Boris' highchair for mealtimes and put it in the garden and squirt some paint onto it and Boris would smear the paint all around and roll in it and generally have a wonderful, crazy, painty time and I would take beautiful prints from the patterns he created on the mat.
However...it didn't quite go to plan, basically Boris was really not particularly interested in smearing he paint around, he didn't really want to touch it much at all. I gave him a brush which he used a little, but I can honestly say he spend about 30 seconds on the mat actually playing with the paint, very disappointing as it took be about 45 minutes to wipe it all up and remove the grass and moss stuck to the back of the mat! And then I had to clean him up because somehow, even though he hardly seemed to touch the paint, he ended up with it all over him.
I am hoping it is because he is still small and didn't really know what to do and/or was a bit frightened.
I did manage a couple of snaps though for the short amount of time when he was engaged with the paint.
Might work with your kids though, perhaps if they are a bit older? Have you tried this with your toddler? How did they react? Did you get some better results than me?