Okay, from Mary GrandPre’s illustrations:
Here’s a Quidditch outfit, showing pants.
Here’s Ron burping slugs while wearing Muggle garb.
Here’s Harry fighting a boggart while wearing Muggle garb.
Here’s Harry fighting a dementor, with robe and pants clearly evident.
Near the kitchen, wearing robes that brush the ground–no pants evident.
Wormtail wearing robes and pants.
The Order of the Phoenix appears to wear robes and pants, though this is not entirely clear at this resolution.
Umbridge wears robes over a dress.
Robes ‘n’ pants.
But Dumbledore and Voldemort, with Dumbledore wearing robes over a robe.
Slughorn attired in–well, I’m not sure quite what.
Draco using a mirror that magically erases the lower half of the body in order to preserve the pants mystery.
Draco and Filch, both with pants.
The marauders appear to wear open robes, suggesting the existence of pants.
Note that Harry’s “robe” appears sometimes to be a cape instead. Note that Snape, Voldemort, and Bellatrix (not linked) wear a different, high-collared style of robe.