Women are very irrational creatures. If she wanted to fight you for custody (don't know if she has full custody) she could simply tell the judge you allow your child around cannabis. You will get boned HARD. They won't care if its for medical use. If its an illegal op you'll get raided.
"hell hath no fury like a woman scorned"
Pissing her off would be bad for you. When women get together and start talking rest assured shes probably already spilled the beans while having a conversation with her girlfriends "Yeah that stupid fuck lets him around his pot plants and they are hurting him" I'd be willing to bet anyone that even remotely knows her knows about your grow now. It's in a womans nature to protect her offspring. When you have new parents they go apeshit when the child nose runs and immediately takes him to the doctor. So, she thinks the plants are bad for him. She's told her mother/father and her friends. Pretty sure of it.
If its an illegal grow I would pull it up, let her know you're not growing anymore and then find another place to put it after some time..You can't trust anything with a vagina.
If you're not wanting to do that, simply tell her that the chemicals hes breaths in while hes outside around cars is worse than the plants. If hes showing no signs of any allergies (red eyes, runny nose, skin rashes) then hes perfectly fine. I suggest moving the tent someone where he won't be.
Everything might be fine and dandy for now..But rest assured you're giving her ammo to use against you when it ultimately comes to that. If you lost your temper 5 years ago and threw something.."Judge he also has a really bad temper"
Find a new more secure spot to grow.