Haha, you know how dangerous pesticides are right? That the people farming the crops won't go in the field afterwards because they're so dangerous??
OP, I did vote YABU, BUT then I read the comments. And I feel I, and a large number of others, have potentially misjudged you.
Unfortunately by telling us in advance that you have OCD, it has enabled us all to fix on that, and basically do exactly what you feared, and call you a hysterical mother. Or at the very least hint at it whilst telling you that you need help with your OCD 🙄 Yep, you perhaps do, hut thats not the issue here. The issue is, did grandma do something that could have been potentially harmful to one or both toddlers, when she likely knows that mum is an overthinker at best. And the answer is yes, yes she did. She could have 'fed' her plants another time when the kids were not there. Hell, she could have watered them another time! I expect she thought it'd be cute and gave no thought to the consequences. Well someone has to 🤷♀️ and this time, as is usually the case, it was mum!
Leaving the open box on the floor was a BIG fucking problem for me too, considering theres TWO toddlers on the premises!! If she thought it was cute to let 3 yr old help, she could easily have put water only in the cans.
Miracle Gro is one of the most popular plant feeds and if you look at their boxed powder (to be made up with water) it has warnings ALL over it, to not allow contact with the skin and eyes, to rinse immediately if any splashing occurrs to ANY part of the skin, and most of all to KEEP AWAY FROM CHILDREN.
So yeah, OP, I wish I could go back and change my vote tbh, but I can't. All I can do is offer an apology for casting a vote too soon! And also let you know, in this situation, with this information, no you were not being unreasonable. You were being perfectly reasonable in fact, as well as classy in the way in which you withdrew, if it went as you tell it anyway 😉
Its your husband's mother, not yours. He needs to say something to her, NOW, soonest possible, not next week, not next month, not even next time you see her. And he needs to take your side, he needs to say we had a problem with it, not wife had a problem with it. When he married you, he agreed to forever have YOUR back. And in this situation, more so than ever@)!
So no, you were NOT being unreasonable. BUT I do also think that a little help with your OCD wouldn't hurt. At the very least it may help you stop second guessing your very right mummy intuition. Xxx