孙子 (sūnzi) is a noun meaning "grandson," referring to a son's son. Note that this word is pronounced differently from 孙子 (Sūnzǐ), the famous military strategist. Context usually makes the meaning clear.
爷爷最疼他的小孙子。
yéye zuì téng tā de xiǎo sūnzi.
Grandpa dotes on his little grandson the most.
她的孙子今年刚上小学。
tā de sūnzi jīnnián gāng shàng xiǎoxué.
Her grandson just started elementary school this year.
老人每天都带孙子去公园玩。
lǎorén měi tiān dōu dài sūnzi qù gōngyuán wán.
The old man takes his grandson to the park to play every day.