There are a couple of things you should keep in mind: is the child comfortable riding with strangers? Would they (and you) be ok if the ride breaks down while they are on it? They should be able to respond to directions that a CM may give them if the ride has to be evacuated. While it's kind of a worst case scenario, it's still a good situation to think about and something you can discuss with your child if they want to ride alone.
Those are very good conversations to have with kids (and parents) heading into the SR line. Since
Disneyland Resort has more SR lines than WDW (as far as I know), we have the conversation with DS about DLR. There are some rides we're all comfy with him going SR (Radiator Springs Racers!) and others we're not (California Screamin'). Different rides, different possible breakdown scenarios, different answers.
The entire point of the single riders line is that you should not expect to be able to ride with your group. If it works out that you're in the same car with someone in your group on Test Track, cool. If not, that's how it works. Groups shoudn't ever expect to stay together if they choose single rider. Im not sure how they could make an effort to split groups up when, in the single riders line, there are no groups.
Going by RSR b/c the seating is similar to TT, even while totally willing and ready to be on entirely different ride cycles, they have put the 2 or 3 of us in the same car or ride cycle. Not next to each other, but near. It's not what we intended at all, but it's their decision. So if TT is trying to make sure that doesn't happen, they'll be more intentional about where they are sending people.
What I meant is they will send ppl who are together to different cars or different lines in the car. Which is annoying for families like mine who want to ride together. We want 3 in the first row and 1 in the back (when we are a party of 4), not 2 and 2 and then single riders in the 3rd spot. It's rude to assign us like that when we've waited in line to ride together.
I bet they think they are doing you a favor, or at least that one person that otherwise would have to ride with no one they know next to them. Why not just say "3 and 1" when they ask how many in your party, instead of "4"? Help them out a little by letting them know up front that one of you is fine riding "alone".
ETA: as a person who is put with strangers a lot (we are a family of 3 and my guys love to ride together), I have to tell you that I am a rule follower, and would be highly unlikely to comply with your request, delmar, for me to move. Don't be surprised if you encounter people who want to follow what the CM tells us to do, not the random guest.