Well, what you do is pretty much complain and pull your hair out everytime she pulls one off, then you end up w/ a tree w/ no ornaments on the bottom and lots on the top, then when your precious child is in bed - you and your husband will look at that sad tree and laugh :)
We finally got out of that stage w/ our kiddos, just this year though - they're now 3, 5, 8, and 11) -- then we got cats and now I can't keep them out of the tree, lol.
When ours were younger, once we put the tree in a playpen, then one year we put in on a card table where kiddos couldn't reach it.
Then we stopped fighting that fight, just put the tree on the ground and anchored it to the ceiling (we put a hook in the ceiling, then tied a string from the top of the tree to the hook so that no kiddo could pull the tree down on themselves - or knock it to the ground).
Even last year my littlest was pulling ornaments off, so all that we could do was smile everynight when we put the kids to bed and looked at our poor bare-bottomed tree and remind ourselves that the toddler years go by so quickly and how truly blessed we are to have experienced them with our four precious kiddos :)
And every time we clean the house for the next few months after Christmas and find an ornament under a bed, or under the couch, it reminds of how much joy our kiddos have each Christmas and keeps the spirit alive for us all year long :)
Merry Christmas :)