There are some shows which are signed, which required no prior arrangement. Much like the interpreters at DL (they all work for the same company), they are assigned certain shows on those days of the week, and will go to those shows whether or not they have been specifically requested.
I believe that if a show is not interpreted and other means of communication are unavailable (such as the handheld captioning device) then you may be able to arrange for that particular show to be interpreted.
At least in DL, interpreters are not available through Disney to be essentially personal ASL tour guides -- that is, the family should not expect for the interpreter to follow them around all day. Though I am certain there are companies that will provide this, this is not something Disney does free of charge.
What deaf or hard of hearing guests ( or ASL students -- the interpreters welcome hearing people as well, so long as the deaf families get the priority seating) should do is go to Guest Relations (in WDW I am not certain if it is available outside the parks or if the guest must go to Guest Relations inside the Park) and ask for the current ASL interpretation schedule. Show times change frequently, so it is best to get the schedule when they arrive in the park.
to the OP, you will want to make your clients aware before they get there about which Park the interpreters are in which day, so they can plan their itinerary. Interpreters wear black pants and a royal blue shirt -- they will have a name tag. The schedule that the guests will pick up in the Park will tell them where at a show or theater they can expect to find the interpreter (i.e. which side of the stage they stand on). It will also give the guests and idea of how early they should expect to arrive for each show. I have found, personally, that if you need reflective captioning or interpretation, that you should get there even earlier than the schedule suggests, especially during the summer.
One show that I absolutely encourage the guests to attend is the Festival of the Lion King - the interpretation of this show is absolutely beautiful.
When arriving at any show, the guests should make the cast members are where that they need the ASL interpretation. The cast members will then direct the family to the correct part of the theater (if the show is in a theater with seats like the Lion King show).
your guests should also be aware that there is a handheld captioning device, which provides the spoken narrative written down for the attraction. I do believe that a deposit is required for this.
Your guests should also ask for a Guide Map for Guests with Disabilities for each of the parks (usually they are available all in one place, when they stopped to get the schedule of interpretation from Guest Relations). I know much of the Deaf community do not consider themselves disabled, but this is still where they will find out whether an attraction has handheld captioning, reflective captioning, interpretation, closed captioning, etc.
As to earlier comments, some people were right that your guests may not asked about accommodations at check-in because the are used to interacting with the hearing world. What you as a
Travel Agent need to make sure of is that they have a Deaf accommodated hotel room, as Talking Hands explained earlier, with a TTY, a flashing doorbell, and appropriate fire alarm safety measures.
As their Travel Agent, you can also tell their airline as well as the hotel and any car service, etc. you may have arranged that your clients are deaf. It will at least give the airline the heads up, though I doubt any changes will be made.
I hope this helps. I have limited experience with the interpretation in WDW, though I go to see the interpreters in DL at least once a week, so (especially you Talking Hands) if I got something wrong, please let me know so I don't make any mistakes again. Thanks