It's training for when they are older have have black, yellow, worn adult teeth.  Yay proper dental hygiene! 

In China, dentaly hygiene is not yet well known. Some of the reasons are:
1) parents or grand-parents don't know about dental hygiene or how to properly clean a a toddler or a child's teeth.
2) baby's toothbrush and also toothpaste are not sold that much and you need to go to special baby shops to find some imported brand (especially Japanese ones)
3) contrary to some Western countries, pediatricians or doctors in China never check a baby/child's teeth to see if they grow up properly or if some cleaning need to be done.
4) for children who go to kindergarten, or later to primary school, no check is done at school on a yearly basis
5) children in China are often given sweets, candies, cakes or soft beverages when they are still young, but without the lack of good dental hygiene, they may later have teeth problems or teeth turning "black".
I know some Chinese mothers whose child is much older than mine who has never brushed his/her teeth yet.

May be young kids like to eat food containing chemical dye.Another factor may be the personal hygiene,young kids should be taught to brush their teeth at least twice a day.

Not every culture obsesses over their teeth like the West does. I've seen people in China whose teeth were covered in plaque so badly that I couldn't see the individual teeth. It freaked me out but they probably didn't make enough money to go to the dentist to do anything about it. Thankfully, most of the Chinese people I know make sure to brush their teeth at least once a day. 

i think there is something fishy fishy in chinese food stuff and secondly they don't delibrately care for their teeths,most of their mouths are stincking.