Oh man I went through this with two cats at our last house. We would buy a tree and they would knock it over as soon as we put it back up. I cried and cried it was horrible. But the way we fixed that was to get a fake tree. Cats are drawn to the smell of a real tree, try a fake one and hopefully all they do is go underneath it to sleep.
In regards to your now walking child. Not much you can do. Put the ornaments on the top half of the tree and pray, lol. You can put a gate around it, have seen it in magazines, but I guess then you don't teach your child anything that way. Believe me one year my son took ornaments and lobed them into the kitchen and I lost a lot of glass bulbs, ugh!
Good Luck!
J. in Macomb, MI