When Walter White was contemplating how he was going to topple drug kingpin Gustavo Fring, he noticed the potted lilies in his backyard. ("End Times") Brock Cantillo was poisoned by Walt in an unknown way with the Lily of the Valley berry. The doctors initially thought he was poisoned by ricin as Jesse Pinkman had suggested but later discovered the true toxin. The doctors and Jesse believed that Brock came across the flower growing naturally and ate some poisonous berries. ("Face Off") After returning home from successfully killing Gus and getting Jesse back on his side, Walt disposed of the Lily plant from his backyard. ("Live Free or Die")

According to the doctors taking care of Brock, Lily of the Valley is not as poisonous as ricin, and they came to the conclusion that Brock might of thought of them as fruits (as they claimed that it would be easy for kids to mistake the venomous plant as consumable), and slipped one in his mouth, causing him to poison himself.

Now the episode makes you believe that Walter White poisoned him, without actual evidence, because he had the plant in his backyard, and disposed of it after killing Gus. Source: http://breakingbad.wikia.com/wiki/Lily_of_the_Valley

So Walter might of wanted to temporarily have Jesse's attention by making him believe Gus poisoned Brock, thus sort of brain-washing Pinkman to get him onboard to kill Gus, without causing real harm to the child.