No, that is very unusual even though you may encounter it in the 19th century poetry. "toho chlapce" acts like a non-agreeing modifier (does not change with cases) and should be placed after the noun.
Isn't toho used for the Accusative case? If I am correct, why is this word here? According to me, it should have been to, since it's the Nominative case (and chlapce the Genitive, yes).
I am sure I am wrong, but please explain to me why, I need to know. Thank you in advance! =)
It is not těch chlapců because we have "the boy's mother" and not "the boys' mother." Since there is only one boy, we need the genitive singular, not the genitive plural. But you might have just gotten matka těch chlapců stuck in your head from another exercise; happens to me all the time! :-)