Our living room has a cathedral ceiling, so we put our tree on a round coffee table... that always helped keep it up out of reach of curious kids....
Anyway... when my son was 3 or so, I allowed him to gently "rearrange" the ornaments on the lower branches.... they were not breakable... (the breakable ones were up high).....
One day, the 8 yr old (she tended to be very bossy, and 3 yr old always resisted HER telling him what to do), came in, and found him rearranging a few ornaments.... she ran toward him, yelling to leave it alone, and tried to pull the ornament away....
Guess what? Yep... the tree fell over.
No real harm done... yes, it did break a few ornaments.... but we put it back together. No one was hurt or anything...
A couple of suggestions...artificial tree (we've been using the same one for over 20 years, now, about 6 1/2 feet tall), and no tinsel..... and put the tree up on a small table, like an end table. Put it in the corner of the Living Room so it isn't quite so tempting, maybe?
ETA: We also have a Christmas Tree climbing cat...... (we have 6 cats, and one loves to be in the tree! The other's have explored it, but she is lighter and doesn't shake it as much as the bigger cats). She hasn't knocked it down yet, either.