MIL issues

i don't know exactly where to post this so i just assume it's lifestyle? 
i need to vent ladies. 
so, i'm married to my husband of 8 years. we live with his parents. or should i say they live with us. we divide everything equally. except for his mom. she pays for food and cooks, while my FIL, my husband and i pay for everything thing else. 
my PIL has 3 kids. (they're all adults now with their own family) 2 girls & 1 boy. (my husband being the middle child and only boy) 
anyways... my son (6 yrs old) has a major allergy to PEANUTS. my MIL would make this sauce for noodles that my son loves. she would also have crushed peanuts on the side for anyone that wants to put it in the sauce themselves in a separate bowl. the only person in the house that eats peanuts is my FIL. but he would use the same spoon that he scoop the peanuts in and scoop the sauce too! so the other day my son ask for noodle and the sauce. so i went ahead and prepared the noodles and i'm about ready to put the sauce on his noodles but then i realize i see little bits of peanuts floating. 😡. so i told my son i'll make another sauce for him and he wouldn't have it.. he wants grandma sauce. so today i told my MIL about the incident and she got the nerves to say "it's just a little bit of peanut, it wouldnt hurt him." OMG  ladies! i was so livid! like are you for real? your grandson could die from this and you're saying it's nothing?? so  we were "discussing" this back and forth very loudly and she got the nerves to say "watch when you're not around, i'll give him peanuts." 😡😡😡 . i flipped out on her. that's when my husband came and told us to shut the F up. i told him that HIS MOTHER wants to kill our son! and he said she's just joking. I DONT CARE IF YOURE JOKING OR BEING SERIOUS, BUT DONT EVER DO SHIT OR SAY SHIT THAT WILL THREATEN MY SON'S LIFE!! like for real.. how am i suppose to trust her with my son now?