I ran into a similar issue with my kid at a similar age. I ended up getting a Polisport Guppy Junior. It doesn’t mount directly to the rear rack on my RR+, so I had to fabricate a mount for it. When I was researching the seat, it looked like it might be possible to mount it to a RR+ with the Yeppi Seat Adapter but I never tested if this was true.

My kid is now 6 and loves the seat. I like that it has a strap that keeps her a bit more stable. Her only complaint is that there are times she wishes she had a handle or something besides me to hold on to.