I believe the original phrase is originated from the Canton / Guangdong region (I could be wrong). While, "Gong Xi Fa Cai" is the Mandarin pronunciation, "Gong Hei Fat Choi" is the authentic Cantonese pronunciation.
The phrase roughly means "congratulations / bless you for some big fat earnings", which was a very materialistic bless saying from the old times. Kids say this to gain red pockets, which contain real money.
Wish you all a Happy Lunar New Year 🙂