Mmmmmmmm, BRRAAAAAIIIIINNNNSSSSS!!!!!!
Strictly speaking, there's no rule that something has to be intelligent to make trip or disarm actions. Afterall, wolves have a special ability related to tripping and they've only got an Int score of 2. Yeah, skeletons are completely brain-dead... (for lack of one)... It's up to you really, but by the books if you can make attack actions you can disarm or trip, etc.