As far as letting an 8 year old sew unassisted, that would depend on the child. I was sewing on my grandmother's treadle Singer when I was 6, after she taught me how to use it. Then later on, she bought a newer Kenmore machine and let me take the Singer home with me. My mother doesn't do ANYTHING crafty, so I was on my own for sure.
It truly depends on the child's maturity. And natural mechanical aptitude.