We have only stayed in a 2 bedroom at SSR, it had a highchair in the closet. I'm assuming the 1 bedrooms have the same and if not, call mousekeeping and they will bring one.
As far as I know, all DVC units have a high chair, usually in one of the closets. They are normally the wooden "restaurant-type" that pull up next to the table, no snap on tray.
We stayed in a 1BR at SSR and the highchair was in the closet off the master bath. We also stayed at OKW 1Br which had no highchair and although we were told "they would have to see if any were avaliable" they brought one to us with 30 minutes!!
High chairs are normally in the entryway closet in OKW one bedrooms, as the previous poster said, if there is no high chair, be sure to call housekeeping.