This was our plan too--kid-led first day--on our first trip when DD was turning 8 yo. However, I did some gentle leading. Since the majority of people go to the right (literally) at
Disneyland, I let DD know this, and suggested that maybe we go left when we can. So, instead of doing Tomorrowland or Fantasy Land first, we did Adventure Land. My DD was (is!) a complete thrill-seeker, so there weren't any worries about her being scared of rides (quite the opposite!). We did Jungle Cruise first (no drops, minor splashes when Ellie the Elephant sneezes, and maybe near the [metal, clearly fake] man eating piranhas, and one gun shot, that they warn you of before the do it). After that, we did Tarzan's Treehouse (which goes high, but provides awesome views, and is completely child-powered--no "ride" just a walk-through play area). Then DD started begging for some "scary" rides, so we did Indiana Jones. DD continued holding onto the "go left" idea, so we ended up in NOS, HM, and (gasp) Splash Mountain. I think I about died on Splash, and she laughed the whole time.
The point I'm really slow at making is that a little "helpful guidance" doesn't ruin the fun--in fact, it may enhance it some. DD's first morning in Disneyland had very minor lines, and, I believe, as a result, she never seems to "remember" the long lines that she had to go in, because I went into it a little more educated in touring, and could suggest where to go to avoid the worse lines at certain times (and, we made use of FPs). True, she didn't go on many Fantasy Land rides that first day, but we still managed something like 20 attractions, with minimal lines (!!) and more fun than she could have ever imagined. ... and she's not a dark-ride kinda kid, apparently--she spent the next two years praying to reach 65 lbs--the weight limit I set before saying she could go on California Screamin' (no, no safety reason behind that, just freaked out mama of a tiny 8yo, and 65lbs seemed so far away when she was only 46 lbs at that first trip!)...
Like others have suggested, consider looking at ride videos and seeing what appeals to your kiddos. ... And think about their personalities overall--if you have a child that that likes to be "warned" before something changes, consider watching the ride videos a few times so you can warn them that (in Indy, for example), "After the big rock comes a quick back up and drop--Oh, there's the rock, we count to 3, 1, 2, 3, hold on! Whoosh! No more drops!" I lived in relative fear that I had forgotten a HUGE drop in Pirates--we went over the first drop, and then the second, and the whole rest of the ride, I feared an unknown bigger/badder third drop that never happened, just because I didn't prepare myself (and DD) on what to expect... Neither of us are good at surprises
