There has been a lot of discussion in the past. Try a forum search using - child baggage, and jumpseat or jump seat
I was one of the first to do this about 18 years ago (
HERE is a post in an old thread containing a picture of him right after arriving at the Arlington Fly-In with his Mom and I in our RV-6A), and I discovered that the biggest issue isn't weight, it is their height. As they grow, they quickly out grow the space for head room and leg room (if seated facing forward).
The leg room issue could be dealt with by seating sideways but that is not a good idea from a safety stand point.
If I were doing it all over again I would probably make leg extensions in the lower part of the baggage bulkhead and mount a high quality child seat facing backwards. This puts their head further fwd (where there is more head room), the window line would be a bit lower, and it would be a safer seating direction in a crash.
Having said that, I probably would not do it again. They grow so fast, it is just not usable for very long. Their is no way I would do it if they had already been born into this world, and I was not yet finished with the airplane. By the time you get it flying, there will likely be very little time left that they will be small enough to use it.