Any kid can ride so long as tall enough. When my son first rode at age 4, my husband sat right behind him, so he could reach out and touch his shoulders in reassurance during the ride. Turns out he's a maniac and didn't need the reassurance, ha!
Barnstormer is pretty small and low key -- I bet they'd let the younger kid ride on her own right in front of your seat. You might even be able to squeeze 3 across with two little ones.
I've definitely ridden three across on Splash and Thunder. I think the only other rides where it is strict 2 across are SDMT, RnR (little ones probably aren't tall enough anyway), and Expedition Everest.