Grandson and Granddaughter are generally signed by fingerspelling G-R-A-N-D
and then signing SON or Daughter.
Some people sign an initialized form of "big" using a "G" handshape to mean "grand." This sign is normally interepred as "great" as in "large." (Not "great" as in "wonderful."
So they are basically signing "GREAT-SON" but interpreting it to mean "grandson."
Personally, I don't have a problem with that sign combination. If people understand it and it spreads then that will become the sign. Some ASL purists might complain though. Hmmm...will it catch on? Only time will tell.
For now I encourage my students to spell G-R-A-N-D when used in "granddaughter."
If you mean "grand" as in fancy...then do the established sign for FANCY.
American Sign Language University ™ ASL resources by Lifeprint.com © Dr. William Vicars