I don't really think your ex gets a say in the matter as to whether his adult children get to know or not.
They are adults and are responsible for their own lives and make their own decisions about everything including their relationships.
He doesn't want anything to do with the baby; he doesn't want his adult children to know; that's because if they DO, then HE is faced with owning up to his actions, his lies being discovered (remember, a lie never stays a lie, the truth is always founded one way or another) AND being put in a position where he can't really say no to being in the child's life.
Try to see it from the perspective of HIM using his adult children as a means to not own up to his responsibility.
If I found out my dad had fathered a child, i would want to know about it. I have the choice of having a relationship or not; my dad does NOT get to make that decision for me.
An acquaintance I know had a child with her partner of 4 years. He wanted her to have an abortion. She refused. He ended the relationship and then 'rigged' his financial and employment status and means of a salary so he didn't have to pay child maintenance. The awful thing is, at the time of my acquaintances pregnancy, he had an 8 year old daughter from a previous relationship whom he still had contact with!
What is so lovely is that the 8 year old's mother was informed by the acquaintance of the pregnancy and the mother encouraged a sibling relationship between the 8 year old and the newborn baby girl. 7 years later, both girls are best of friends, see each other regularly and the then 8 year old girl still saw her father DESPITE his refusal to acknowledge his younger daughter.
Lately the older girl has opted to cease contact with her father citing him to be a 'dead-beat dad' who refuses to face up to his responsibilities and has vowed that until he accepts and acknowledges his younger daughter, he doesn't get to see his older daughter.
My cousin was also an unwanted child. My aunt (had 3 sons by previous marriage) also had a DS aged 2 when she fell pregnant with DS5. Her exDH threatened her, physically assaulted her and then kidnapped DS4 whilst she was pregnant. 25 years later and her exDH STILL to this day refuses to acknowledge DS5 but happily has contact with DS4. No wonder DS5 of y aunt is seriously FCUKed up.