At Universal everyone can go into the line. You and the 6 year old would just exit at the sorting hat, and the 9 year old would continue on.
Also Universal does it differently than WDW, in that there's no *age* for being able to go solo. Rather it's a height. As long as the child meets a certain height requirement (AND is tall enough for the ride...the height req for being solo is shorter than, say, the Hulk's height, so you can't go on a ride with a higher height just b/c you're of the height to ride other things alone) they can go alone.
Right now I"m trying to jog my memory of the height. I KNOW that on our first trip to Universal, in dec 2010, DS was 6.5. We had gone on Flight of the HIppogriff 14 times in a row and were finished with it (b/c I couldn't take it anymore and it was like 29 degrees that morning and my face was nearly frozen off) and I noticed the sign about how tall you have to be to ride alone. DS was already at that height. So when he wanted to go on Woody Woodpecker's Nuthouse Coaster 15 times in a row to end the day, he went ALONE.
Ah yes, there's the wording.
Why do you have to be over 48 to ride alone?
The Universal Orlando Resort has a strict safety policy stating that if a person is less
than 48 tall they have to experience many attractions with a supervising companion. A
supervising companion should be an adult or older sibling.
So it's 48".
Hippogriff's minimum height is 36". Once that child reaches 48" they can ride it alone.
But Rip Ride Rockit's minimum height is 51". A 48" child wouldn't be able to ride it at all. But once they reach 51" they can do it alone.