In the campaign I'm running one PC's niece was kidnapped and eaten by a hag. When the party found her she had been "rebirthed" as a hag. Basically hag-daughters look like normal until their 13 birthday, when the become a hag, which I'm interpreting as having the same personality/mind until 13 and then becoming evil like a hag.  To make it worse, the niece is 12 and her thirteenth birthday is in one week, so the players are in a rush to find a cure. Is there any canon way to cure a hag daughter? If not, what would you rule it to be?

Unfortunately, no. Hag-changelings aren't children; they're young hags with the soul of the child. At 13, they shed the child's soul and change their body. If they somehow managed to get immortality and/or stop her from aging, then yes, that would be a "cure." The only other way I might approve of is killing her and casting reincarnate because her soul would take a different body. 

My 2cp worth: Treat it like a Bestow Curse spell cast with a 9th level spell slot and have the party research/find a ritual to remove it. The basic ritual could involve casting the Remove Curse spell which halts the progression to a Hag but results in the child being a Hexblood (see Van Richtens Guide to Raveloft) and/or becoming a warlock bound to the hag, this then buys the party time to find a way to remove the "taint" of the Hag such as: killing the Hag and bathing the unfortunate child in Hag's blood or brewing a potion in the Hags Cauldron and then bathing in it or going full on Brothers Grimm and having the Hag pushed into her own oven and baked into a pie which the child has to eat.

Quote from Rob76 >>

My 2cp worth: Treat it like a Bestow Curse spell cast with a 9th level spell slot and have the party research/find a ritual to remove it. The basic ritual could involve casting the Remove Curse spell which halts the progression to a Hag but results in the child being a Hexblood (see Van Richtens Guide to Raveloft) and/or becoming a warlock bound to the hag, this then buys the party time to find a way to remove the "taint" of the Hag such as: killing the Hag and bathing the unfortunate child in Hag's blood or brewing a potion in the Hags Cauldron and then bathing in it or going full on Brothers Grimm and having the Hag pushed into her own oven and baked into a pie which the child has to eat.

 I like this idea. I'll probably do that. Thanks for the suggestion!

Quote from MaxGnomeDruid >>

In the campaign I'm running one PC's niece was kidnapped and eaten by a hag. When the party found her she had been "rebirthed" as a hag. Basically hag-daughters look like normal until their 13 birthday, when the become a hag, which I'm interpreting as having the same personality/mind until 13 and then becoming evil like a hag.  To make it worse, the niece is 12 and her thirteenth birthday is in one week, so the players are in a rush to find a cure. Is there any canon way to cure a hag daughter? If not, what would you rule it to be?

 If they could find a way keep her from being evil, than being a hag might actually be pretty awesome. You get some neat perks!

If you want to take another route, you could make her become a Hexblood.

Off topic, but I'm thinking of having a hag NPC eat a child in my campaign, but I'm not sure how to go about it. Kidnapping? Magic?