Is it permissible for my mother to donate the value of the utensils she took from my grandmother without her knowledge, given that she knows her grandmother has other children?

Your mother's conscience will not be cleared by donating the value of the utensils she took from her mother's house without her knowledge. She must return them to the estate or seek permission from all the heirs. If she is embarrassed to tell them that she took them without her mother's knowledge, she is not obliged to inform them of that. It is sufficient to return them to them by any means that does not cause her embarrassment or lead to harm.