If they do, and they can know or understand about poison and unknown plants then you can teach them and tell them. You can also kind of identify for them which plants are safe and which are poisonous so they can eat some maybe. The big problem of course with neighborhood plants is that some, even if they are otherwise edible may have pesticides or who knows what awful noxious stuff on them.

If your kids are too young for that kind of teaching or explanations and will imitate that 9 year old you might have to cut off that contact. Don't let them play with the child who has the problem behavior which could be a danger.

if you have a yard or wouldn't mind growing some herbs in a window you could grow special plants that they could learn were edible and they could have ones they could eat or make salads from or whatever

I don't know how old your child is, but if they are that much younger than the 9 year old they probably shouldn't be playing with him without an adult around anyhow. We had a neighbor who was 12 and my 8 year old and the 8 year old neighbor wanted to play with him all the time. The neighbor's mom would let her son go into the 12 y/o house to play anytime. I said NO to my son. I told him he could only play with the 12 y/o if it was in our yard and I was there. I told DS, sorry, but your rules are different that a 12 y/o's rules, you need to be with kids your own age with 8 y/o rules. That 12 y/o could have been home alone for all I knew.

If you kids are the same age, it's time to talk about peer pressure. That you can't say "It was Jimmy's idea" and expect that to get you out of trouble. Use you own brain, make your own choices. Explain to your kids the chemicals that could be on those plants, and that some plants are just poisonous in themselves. Since they don't always know the difference, don't eat what isn't food! Explain the potentional consequences of dangerous plants - nausea, vomiting, brain damage, death.

Over 95% of plants have parts - flowers, leaves, seeds, roots, etc. - that are toxic to humans. Because plants can't run away from predators, they develop toxins in their tissues to keep from being eaten by animals.

It is extremely dangerous for children - or anyone else - to eat plant parts unless they are certain that they are edible. A couple of years ago we had neighbors who let their two-year-old "graze" in their yard, because they believed that it was "natural" behavior. The little girl wound up in the hospital after eating toxic ivy berries.

Amygdala and Stephanie also make good points about pesticide residues on some plants.

Whatever it takes, don't let your kids become uninformed "grazers." And although I'm sure it was annoying to have your neighbor laugh at your concerns, for her nephew's sake try talking with her again. Print out the responses to this post and give them to her.

Good luck!

Best,
Elizabeth