This is something that's always bothered me. Regina just says "don't tell anyone about me and Daniel" but doesn't tell her why she should keep it a secret and that her mother was psychotic

She kind of did...she said her mother would stand in the way of her happiness, that should have been enough. She was old enough to know to keep a secret in the first place. My little sister is 8 and I know for sure she wouldn't do that.

Cora has manipulated kings and sorcerers. The poor kid didn't stand a chance.

A little kid is very easy to fool and take advantage of. Regina should have known her mother would be suspicious and should have warned Snow that Cora might try to trick her.

I feel like she kind of did. I also feel like she didn't want to scare Snow which is why she just said 'don't tell anyone. And let's face it, if Snow was afraid of Regina's mom and Cora picked up on it she probably would have played off that fear to get info...maybe even resorting to violence. So in a way she kind of protected Snow.
But then again it could have worked. So I picked not sure.

@Fieryrubes I agree with all of that. I know a bunch of 8 year olds who can keep a secret.

Haha and Snow was like 11 or 12 :/ Yeah exactly she did warn her. Anyway Daniel's dead, Regina's good let's move past this. No one really misses him greatly.

Right, if you think about it, she was almost in high school. A teenager. Teens are know for 'lying to their parents'.

Did Regina knew what her mother was capable of though?

Yes, she said that to Daniel. ^

^ her mother also used magic torture on her

I wouldn't call it "torture" but magic.

Snow had no real idea why Regina might want to keep her relationship and engagement a secret from Cora, except maybe Cora disapproving of Daniel's social status. She would never have guessed that Cora would kill him if she knew. Though I'm not sure if Regina thought the danger was that extreme, either.