She got dumped on a desert planet with nothing. If that's Luke's idea of parenting we are in serious trouble.
That said, makes sense. He was training Jedi, his nephew turned to darkness under his watch and in order to protect his child he hid her on a secluded planet shortly after he vanished from the known galaxy.
If his wife/mother of Rey was pregnant when Kylo fell to darkness or if she was an infant its possible Kylo didn't know about her. Luke might have sensed him falling and kept Rey secret, much like how he was kept secret from Vader. It worked once, it will work again you know.
Granddaughter. They mentioned it during the bar scene.
That would make his child a teen mom at the very best. Its only been 30 years since Endor and he wasn't hooking up a lot in that time period.