This is definitely an outdoor project! Protect the kids' clothes well (or take them off on a summer day!) and be prepared to hose the children down well afterwards.
You will need:
A squeezy bottle for each paint colour (we used water bottles that have a sports cap, but washing up liquid bottles would work well)
Large sheet of card
Lay your card on a bit of flattish yard. If it is windy weight down the corners with pebbles.
Pour the paint into the bottles and add some water to each. Shake the bottles, making sure any lids are tightly closed!
Spray the paint over the card. Keep adding more colours until you are happy with your picture.