Christmas trees are not safe for cats! Real ones are toxic if ingested, artifical ones will cause intensinal blockages if eaten.
Spraying them with bitter apple spray or diluted citrus oil can help, but really the best thing to do is not leave a cat unattended with a Christmas tree. Shut them out of the room overnight/when you're out.
Tinsel can cause intestinal blockages and is also toxic, because it just has to one-up everything else.