My mother has a dog with aggressive tendencies. I have a two year old son who loves to spend time with grandma, but I'm afraid something might happen. Are my fears justified? Should I request this doe be muzzled around my son?

My mother has a female Yellow lab that she rescued when it was a couple years old. She drove by and saw it outside in a kennel and she was severely neglected. My mother has nursed her back to health and given her a very loving home for the past several years. She has socialized her the best she could. I believe that due to the lack of early socialization and training, she gets very aggressive (only barking and backing up) at some men (not all) and is very aggressive with other dogs at times. She has attacked 4 dogs she lives with in the past 5 years all requiring stitches and 3 near fatal (1 I broke up with my beagle). She has been to trainers and she is very well behaved besides these aggressive tendencies. I have a two year old son that loves spending time with grandma but am afraid that something may happen and I'd never be able to forgive myself for ignoring these signs. Are my fears justified? Do I need to request she be muzzled or just not have my son around her?