Our two 3 and a half month old kittens climb our xmas tree.. how do we keep them out?

we have two three and a half month old kittens. we put our artificial tree up with nothing on it, to give them time to get used to it. they still climb it. i have tried the idea i saw about putting double sided tape down, but i'm not sure yet if that works. they are sisters, and what one does, the other does, too. how can we get them to stay out of the tree so that it's safe to put our ornaments up?