Thanks maybe the end of the night is another good option. That age is 7. The only reason I know this is because they started enforcing it last year and they wouldn't let him ride with his 8 year old brother on TSMM when he was 6 and told us he had to ride with an adult until he was 7. Just wasn't sure if it would be "appropriate" to do in a single rider line or if anyone has experience with kids and SR line.
My nephew was 8 when we tried the SR line, and he panicked when we were assigned our rows. Fortunately we were next to each other, and the person I was assigned to go with was a dad and quickly realized what was happening and swapped places with him. I would never try it again at that young age, we got lucky with how it played out and I wouldn't bank on that happening again! I would have exited with him if we couldn't have swapped.
Depending on the time of you FPs, hopefully you could get a 4th at RnR for later in the day! Otherwise later in the night is your best bet if you can't do RD.