3-year-old Taeyang was diagnosed with b-cell quite lymphoblastic leukemia last week -- a type of blood cancer that's more common in young children ... Deavan's publicist, Lindsay Feldman of BrandBomb PR, tells TMZ.

He's been in the hospital ever since and is expected to stay there a while ... he's already started chemo treatments and is awaiting further procedures. Deavan was told this will be a long two-year process, with a high success rate of full recovery in five years.

According to the GoFundMe page started by Deavan's mother, Taeyang took countless trips to the ER before a specialist told them he had cancer cells. The family is looking to raise $50K to cover expenses.

We're told Deavan attempted to contact Jihoon about their son's diagnosis but has been unsuccessful ... he has her and her family's numbers, emails and social media blocked. She even tried contacting Jihoon's father but was unsuccessful.