Here is what I used
They might not necessarily ask you, but its always safe to have the below documents.
In our case, they asked us once and the other time, they did not care.
1. Copies of Passport of both the parents (page with picture and name details)
2. A letter authorizing your in-laws to accompany your daughter.
Get it notarized if you can. If not, send the signed copies.
If you need a format for the letter(nothing fancy, was drafted by myself), let me know, I can dig out the one I used.