Meaning:
“别人家” means “other families,” and “孩子” are “kids.” Literally translated as “other people’s kid,” this term actually refers to those who have been excellent in many aspects (especially academic performance) since childhood and are often mentioned as role models by other parents to spur their own kids to work harder. This expression jokingly reveals the fact that many Chinese parents are keen to compare their own children with other kids, and not very good at seeing the merits in their own offspring.
Example:
A: 这次中学同学聚会你见到当年的校花了没？
Zhècì zhōngxué tóngxué jùhuì nǐ jiàndào dāngnián de xiàohuā le méi？
Did you meet the campus belle at the gathering of our middle school classmates?
B: 见到了，她还是那么美。听说她在一所重点大学已经评上教授了。
Jiàn dào le，tā háishì nàme měi。Tīngshuō tā zài yīsuǒ zhòngdiǎn dàxué yǐjīng píngshàng jiāoshòu le。
I did. She’s as beautiful as before. I’ve heard that she is already a professor at a key university.
A: 她从小就是别人家的孩子。
Tā cóngxiǎo jiùshì biérénjiā de háizi。
She’s been outstanding since childhood.