Newbie. I have a nice DSLR that absolutely cannot get stolen because I'd never be able to replace it. I want to bring it for some great pictures, but will it get stolen if I leave it with the stroller?? Not sure how this works. TIA!

I would absolutely NOT leave it with the stroller. I wouldn't want to take the chance at all...
I have a nice camera myself I purchased a nice padded camera case thats large enough for my extra click and shoot camera, larger camera and a few extras (tickets etc) when I got on the rides I put the case on the floor and put the strap around my leg. Then it will not bounce out of the ride and I will not forget it in the ride. I take it on every ride, ROR, TOT etc Splash you have to put it in a bag so it doesn't get wet.
What I did last time I went (and will probably do again) is bring my one DSLR-like camera and a point n shoot. The DSLR stayed out ALL the time, unless there was a need to put it away. I carried a small padded camera case in the bottom of my standard-size backpack that I could put the camera into when I needed to. Then, I could stick my leg through one of the straps (like on BTMRR) and rest the bag on the floor of the ride.
For my little point and shoot, if I can feasibly take that onto a ride, I take it! I put the DSLR in the backpack (as noted above) or give it to hubby if he's not riding, and hold onto the little camera. I'm, um, endowed with a big, um, set,

and found that I can tuck my camera into there--worked out great on Splash!
i keep mine with me at all times. I have a nice padded case that the camera and batteries and a couple memory cards fit into. I don't bring extra lenses, etc. I keep it around my neck or sometimes cinch it up and keep it around my waist.
Also remember that many of the lines / waiting areas are themed and you may want pictures in there also.
I found that I dnd't put my camera away until I was pretty much Next in Line, if I had to put it away at all.
Bring a nice large zip loc bag to put it in for wet rides. We got on Kali and there wasn't enough room to put our backpack in the center holder, so it had to go in between my feet.
Got wet of course, but no damage luckily.
While your standing in line, but your cam in the plastic bag and put it in your backpack and your set.
For rides like Splash and Kali, we found it easier to put the ENTIRE bag into a Ziploc Big Bag, rather than worrying about what was inside the backpack (cell, camera, big camera, watch, batteries, etc). The bonus of the ZBB is that if you have more than one person with a backpack, they will ALL fit into one bag, so after Kali is over, just walk away with your bag. You won't even have to worry if the little bag holder in the middle is full--your gear is safe!