Btw - the cling film one might be a bit too old for that age group - but if you do it, you have to let the children "squidge" the paint under the cling film (the paint has to be watery) and then let it dry (on radiator good) with cling film still attached before peeling cling film off to reveal interesting patterns and colours etc.
Another good one is to get a role of old wall paper and use the back. Get one child to lie down and keep still (!!) and then let the other children draw around the child. Then let the other children draw or paint in the features and clothes etc.
A variation of this would be to ask children to draw/trace around their hands and feet on brown paper bags and then cut out. Or draw around hands and cellotape two sides and make "puppets" with faces.
Threading book-marks: cut 12 pieces of sturdy cardboard (from cereal packet)and punch holes in all around. Ask the children to thread stiff-ish string (or shoe laces good because of stiff end) through the holes (again may be for slightly older group).
Get clean yoghurt pots and punch a hole in the end of each. The children then decorate the yoghurt pots how they like - with stickers etc. Ask the children to thread a knotted piece of thick-ish string through the hole. Tie a big bead to the bottom of each piece of string. Then ask the children to "catch" the bead in the pot. This works with children 2 years and up.
Hand tulips are good for spring. Either do them individually or on the back of a long piece of wallpaper. Get the children to paint one palm with bright red paint. The hand prints become the tulip flowers. Then either stick on green string, or use paint to create stems and either green paint or cardboard shapes for leaves.
Food mosaics. Use icing for glue. Use cereals, crackers, dried fruit, minestrone, alphabet spaghetti etc. to make mosaics on paper plates. Paint with the icing!! Use can use different colors of icing if you like.
Another messy idea (but has to be well supervised) is to let children play with some sort of foam (hypoallergenic shaving foam a possibility but probably some other bubbly foam from shampoo that doesn't hurt eyes best). Free play with paper cups, plastic blunt knives, plates and sponges - they love it!!
Tie dye butterflies are good using runny paint or washable felt tips. You just cut 12 butterfly shapes out of coffee filters. Let the children colour them in, add water with paint brushes, and watch the colours bleed together.
A variation on this is "ink blots" using washable paint instead of ink. Just get twelve pieces of good quality thin white card and fold in half (with a crisp fold). Ask the children to paint on just one side of the paper with strong but fairly wet washable paint. Then ask them to press both sides together. They love opening them up to "reveal" the pattern.
Pringle shake: take a clean, empty Pringle's can and put inside a sheet of white paper. Soak several marbles or 3 cm pieces of sponge in several colors of paint. Put the lid on and let the children shake.