Well I felt already that I've been too explicit here. I know that must be very important matter for you but I don't have any negative memories connected to all this.
My mum certainly knew what masturbation is, but she was never abusive and never sexual towards us kids. I'm coming from touching and very warm culture comparing to US which is on opposite side of spectrum (the coldest non touching culture as I know), we kids and our parents never have any confusion about sharing emotions or touching. It never includes sexual feelings and we (as kids and later as adults) are never in doubt about it. It is much different environment than what I can read here, much safer and much more human, but off course abuse is happening here also. So this is frame let's say, my mum tried to be very protective and to help me as much as possible, so she gave me instructions and after some time checked if I tried it and asked if I had problems, she did it with caring and love, she was concerned would I have health issue later in my life.
She told me carefully to do it once or twice at time. And as my foreskin was not stretched I couldn't do it easily and not like masturbation. With time I figured that it become easier for me to do it.
"Phimosis, from the Greek phimos, is a condition where, in men, the foreskin cannot be fully retracted over the glans penis."
In such cases during erection or sex foreskin can crack which could be very painful. I know for my friend who has had such condition and he was in some problems because of it.
Beside all that, later and non connected to this story I've learned to masturbate.
I hope I answered to all your questions.