SDD is deceptively fast (not fast by rollercoaster standards but by Disney standards). At 40mph, I was surprised how quick it felt. The launch could be a little intense for a kid who is new to rollercoasters. The order I took my son on the Disney coasters was:
7DMT (3.5 yo)
Splash (3.5yo) (I know, not a coaster, but included anyway)
If Slinky was around when he was there at 3.5 I would have taken him on it, but now that I have ridden it, I think BTMRR is less “intense”.
7DMT is really short in terms of the amount of “rollercoaster” time.
Another factor that may help is that for kids, sharing a lap bar with an adult does not feel safe to them, since the bar doesn’t come close to their lap (unless the adult is seriously tiny, but that’s not me!). 7DMT and SDD have individual bars, so not problems there. Space is obviously individual bars, but my son is skinny and says he wished the bar came down lower!
But after all that, I don’t know your kid and what they can handle! Mine was a little scared each time he went on something bigger/faster, but I tell him every time rollercoasters are supposed to scare you a little, that’s what makes them fun!
I’d say go with 7DMT just because the mine scene is worth seeing and the ride is smooth and Barnstormer is just a pain for an adult to squeeze into the seats for.