There is something about the ride vehicle design that allows the under 6 to ride with a under 14, provided a over 14 is in the next vehicle. That’s why is appears to be inconsistently applied. It’s not actually. It’s the ride vehicle differences. I admit I can’t cite the differences myself, but I’m positive that someone here can. All I can recall is it’s something about the way they are connected to each other.
In OPs case it won’t matter as her’s are over 7. But for others with someone 6 and under, you have to learn the various vehicle types to learn which they can ride together and which they can’t.