It's true that in certain part of Thailand children are purposely being physically mutilated and force to?

become beggar so that tourist will pity on them and give those children money

Why do you never give anyone a best answer.

I live here in Pattaya, have never seen even one child begging on the street. There are beggars, however far less than say, Mexico, but most are older, and yes, some are handicapped. I spent plenty of time in Bangkok, as well. In the touristy areas, like Silom, and can't think of ever even ONCE TIME seeing physically mutilated children begging on the street.
For the most part the folks who live here, even in the poorest areas look far, far healthier and happier than in the poorer neighborhoods of the USA. I'd say someone is feeding you some information about things that can and have happened, but making it sound like the norm. It isn't.

Yes it is true, but it is nothing like to the extent it is in India.
India is famous for this, its a freakshow.
You will see some of the most disgusting things that will haunt you for the rest of your life.

These beggars in Thailand work for the Mafia and most are Cambodian, Burmese and Laos and most are forced to work.
The children receive very little from what they earn the mafia takes a significant %.
These type of beggars are found in tourist populated areas.
Pattaya has many child beggers (I have not seen any mutilated children yet) that can be found in the gutter of 2nd road on Friday, Saturday and Sunday nights.
Bangkok has Sukumvit rd.

Its very sad.