Answer: As you've found, many common plants are toxic if ingested, and certain plants can also cause skin reactions in people who are sensitive to them. Unfortunately, if you try to exclude every possible toxin you will limit your plant selection drastically. To be honest, I am not sure you could exclude every single toxic plant or potential danger and still have a landscape! For example, tomato leaves are toxic, though the fruit it is safe to eat, and although they may sting, bees and wasps are a necessary part of nature.
It would certainly be wise to exclude the most poisonous plants, such as lily of the valley, and keep thorny plants away from easily accesible areas. It's also key to limit toxic chemical use in maintenance of the landscape. This is a great opportunity to take an educational approach and work from the concept of teaching children only to eat plants that have been identified by an adult or by a sign as edible. In my own experience, this is a part of lifelong learning and a valuable lesson that helps children develop common sense in a supervised environment.
You might also want to discuss this with the staff at the youth center and see how they feel it is best addressed. All the best with your project!
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