We did it with our 9-year-old daughter last week. No problem whatsoever. We also did it for EE. Just tell her where to meet you, and to not leave that area under any circumstance. It works especially well if you have two parents riding - have one parent in line first, then your daughter, then the other parent. That way, there will be sure to be a parent around when she both enters and exits.
When we rode, my wife was in the first car, daughter second, and our son and I ended up in the same car.
Our daughter hates TOT, so the two of us rode RnRC single-rider last year, when she was 8, while my wife and son rode TOT. Also not a problem.
Also worth noting is that, while you can't do the full car design stage, if you rode it earlier in the trip with either SB or FP, and didn't use the limited design stations in the single-rider queue, you can still load up your original vehicle before boarding.