Hi, sounded like an 8 spline crank to me. Does it has a bolt or two right next to the splines?
After you got the spline type right, need to check the spindle size; not sure abt 8 splines, but the other type (48 spline) comes in 3 diff diameter now. Then comes the crank length. It's normally between 165 to 180mm, rider's preference. Kids might like shorter ones, and less chance to get your heel hitting the peg if you do run that. Happy hunting!