As others have said, it's pretty tough to escape seeing Jack Skellington in HMH. I would watch a
YouTube video to get a sense of the ride and whether or not you're little one would be freaked out.
[SPOILERS for the ride - albeit most of this is "known" because it's the same from year to year]
-Jack himself appears at the end of the stretching room. This year uses video projections, which can be kind of scarier than the old static face.
-There is a portrait of Jack in the hallway. He alternates between his black outfit and the Santa suit. He's also nominally visible in the first portrait (Santa sled).
(These are from last year and they've changed slightly with the addition of the Zero-effect, but the main idea is the same.)
-There is a portrait of him in the attic. I think it's on the left side, near the end.
-There is a full sized audioanamatronic figure at the entrance to the graveyard. It's where the gravedigger is normally located. This one you really can't avoid seeing because the cars all swing to face him.
-He's visible (in outline) in the Santa sled in the moon by Sally on the escalator ride up.
-There is also a scarecrow in the line queue area (outside). It's not Jack, but it has kind of the same build.
-There are also figures of him outside on the entrance to the ride.