I agree with Oregon Blues. The chickens are being tormented by being chased by the child. Some people may find that cute, but I don't like to expose my animals to any unnecessary stress just for the sake of giggles. Especially with the hot weather that we've been having in a lot of the country, that is certainly not in the birds' best interest.
Despite how advanced most owners believe their children to be, no 18 month old is aware enough to be trusted alone with animals. It's still not to early to begin to teach him responsibility though, and that begins by not allowing him to chase animals for fun. I have seen turkeys be attracted to children's eyes, just at their own eye level, and attempt to peck at them, as someone else noted. It's not impossible to keep turkeys while having small children, as many responsible parents who are also poultry owners do. They simply keep them seperate, or supervise the children closely when they are with the birds.