Wiz Khalifa -- Amber Rose is So Cold She Denied Our Kid His Birthday Party!

Wiz Khalifa threw a big party Saturday for his son's birthday ... but the rapper says his 2 year-old never showed because ex-wife Amber Rose denied the kid a good time out of spite.

Wiz posted sad pics of the party with the caption, "only thing missing was Bash" -- referring to their son Sebastian.

Sources close to Wiz say Amber was supposed to bring Sebastian to the party and they would celebrate his birthday together ... as parents. Wiz says everyone was there and he's heartbroken the boy couldn't celebrate with civil parents.

Khalifa put Amber on blast, saying, "ive never voiced my true unhappiness out of fear of the person I was with and what she would do to make me more unhappy of anyone knew."

Wiz goes on to champion father's rights, saying, "Standing up for all fathers who need, deserve, or can't find help," adding, "As a man, and a dad you'll feel like, 'I got this.' But who got us?" In other words, the system doesn't protect fathers.

For her part, Rose posted videos of their son before and after the party.