I cannot speak on the safety of buses in that area. Near Guaymas they have robbed not only money but jewelry. Some of the Mexican women keep their money in their bras. I guess they don't strip search. I went from Oaxaca to Puerto Escondido last June in a nice Mercedes Benz bus.
Drivers are very professional in their white, stiffly starched shirts.
There are assistants to do the menial tasks. One takes 6 hours and one takes 10 hours because it goes around through Hualtalco. That's the one I was on, and actually, it took 12 hours because we got a flat tire. You should have no problem with the child, because as you know Mexicans love children. I also took a bus from Puerto Escondido to Mexico City at night.