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Using craft pom poms (or cotton balls!) to paint is a great sensory activity because it gets your child used to touching soft, wet, slimy textures while still being fun and creative.
For less mess, try using an art apron, but keep in mind that not every kind wants to wear one (and if you are doing these sensory activities, your kind might already be extra sensitive to things like wearing art aprons!).
If you choose a nice combination of colors and give your child a canvas, they might actually create something worth hanging up (if you like modern art that's open to interpretation). But think how proud your kid would be to have their art hanging on their wall!