Different coloured dried pulses(or rice with food colouring on)... In different pots so mindiee can choose what colour they are using.
Cut a square (could be any shape) cardboard frame, stick the frame to some clear sticky back plastic and cut around the outside edge, leaving a clear sticky square inside the frame,
Attach the frame to the table sticky side up, (I use weights on top and or blue tack underneath) to stop mindiee picking the frame up.
Mindie can place pulses on top making a picture, if they have left any space all you need to do is push another piece of card on top of their picture giving it a background and stopping anything falling out. sometimes a couple of staples or some double sided tape is needed around the non sticky frame.
Leaves kids clean and tidy, but your floor will need a sweep!!!

This method can be used for framing loads of other things too, leaves in autumn, petals in spring, or photos that you want to be a bit more durable. Or any picture to be made into posh calendars or cards for parents.

This takes a bit of prep on your part so might be difficult as a spur of the moment thing with a lot of kids.
Hope this helps and I have explained properly,

Jesse x