Same two characters, two different words. With a neutral tone on 子 this is sūnzi, a grandson. Read Sūnzǐ, with a third tone, it is the ancient strategist Sun Tzu, where 子 is the honorific Master.
usage
Chinese sorts grandchildren by which parent they come through. 孙子 and 孙女 are your son's children; your daughter's are 外孙 and 外孙女, with 外 marking the outside line of the family.
