Quick note, since it just worked for me. (They had been chewing tinsel, smacking decorations and knocking over the entire tree for two days straight.)
1. Take a small rag (or similar) and ensure you can make it inaccessible to the cats.
2. Add a couple of drops of tea tree oil and place the rag well inside the base, or some place the cats can smell it, but not get access to it.
3. Win! My cats keep walking over to the tree for some fun, then they get a whiff of the tea tree oil, they screw their noses up and back away. And it smells far more pleasant than any other cat deterrent I've ever used.
Tea Tree Oil is probably the world's best natural antiseptic. You can use it on cuts and scrapes to kill germs. If your kid gets the chickenpox, put a couple of drops in a bowl of water and wipe all over the skin after a bath (or just put some in the bath). This will not stop the sores but it definitely helps prevent scarring because the sores are unlikely to get infected when the kid scratches.
IMPORTANT: this stuff is POTENT and it's toxic. Keep it well away from eyes and mouths. And ALWAYS mix it well with water before putting it anywhere near skin. I buy a version that is blended with carrier oil. If you buy pure oil, make sure it's never used undiluted. You also need to use this in a way the cats cannot eat it, just in case your cat is psycho and not deterred.