We took our daughter last year when she was 5. We told her the day that we left, mainly because we didn't want her to ask 1000 times everyday for weeks and also because we didn't want to tell her and then have her be disappointed that we weren't going right away. Now, this wasn't here first, or second, or third trip so maybe others would handle it differently.
As far as 40" rides go, she had already been on Splash the year before, and she loved that dad got soaked, so Splash was an east sell. SDMT is, in my opinion, the perfect intro coaster for young kids. It's short, and super smooth. From that, you can move on to BTMR and Barnstormer if it goes well. The only ride we had to "push" her on was ToT. She was scared (rightfully so!) but we waited outside the exit for other kids her age and talked to them about what they thought and if they had fun. Doing that really helped, along with the promise of something from the gift shop to celebrate. She went on, and it was her favorite ride of the trip.
I have strategically shown her videos or mentioned things about WDW weeks and months ahead of time to A) put the idea in her head, and B) get her familiar with stuff. I don't think it's a bad thing to do that, and I would suggest doing that. Like anything in life, it can go either way, but the upside is enough to give it a shot.