Having thought about it more, I think Brock didn't get the ricin cigarette. I think he's sick with something else, the flu or something equally mundane. When Jesse flipped out and told Andrea about the ricin, Andrea freaks out and tells the doctors, who test for it and it's not ricin. When he comes back to the hospital, Andrea and her Mom want nothing to do with him because he's a delusional junkie, not because he had ricin. It seems to me that if it was ricin, they would want to talk to him, and the police and/or security would pounce on him the second he showed up. Jesse probably has the pack of cigarettes with the ricin in a different jacket, or something equally simple. Given his current state of mind, grabbing a different jacket on the way out of the house seems likely. Occam's Razor.
That makes for a far, far simpler explanation than "Gus saw them make ricin on the video, knew exactly what it was, knew exactly where Walt and/or Jesse had hid the cigarette, had someone break into Jesse's house or had Hule (who's an idiot who works for Saul, not Gus or Mike) lift it from Jesse, track down a 6 year old boy, poison him, in the faint hope that Jesse would flip out, blame Walt and kill him."
If it's neither explanation, I imagine it's far closer to the simpler explanation than the elongated version. The show is far too well written, IMHO, to try to string together a series of extremely unbelievable events. It also fits the show's writing style that this series of endgame events would be set in motion by a 6-year old boy who caught a cold.