If we were talking about a stranger stealing that money from her I would agree with you, because we must do what we can to protect our money, however it was her own son who did this, meaning that a person she trusted, broke that trust by stealing from her own mother, and while the amount was small, what can be more low than stealing from your own mother? So I would not blame her at all for this situation as it is clear that it is the son that is at fault here.
My vision is different. The only one to blame here are those who raised and educated that kid. If he was taught properly about what crime is and how to get money at his age, I think he would not acted like that. Not saying that kid is completely innocent, but there were reasons why he did this. It is not like he learned how to steal himself. Someone showed, told, explained, or lack of explanation what is good or bad had lead to such outcome as stealing from family.
Many children steal money from their parents, and here it is not the parents' upbringing, but their social circle. I believe that most likely someone pushed him to this deed, and he by virtue of his naivety only followed the advice. The child in any case is guilty of taking someone else's money without asking and if you do not punish him immediately it may turn into more serious consequences in the future. If the parents are to blame, they are only to blame for not controlling the child's social circle, which in principle is quite difficult to do before the emergence of such situations, as well as for not noticing the theft in time and not stopping the child before he spent the money.