Nem tudo precisa ser mostrado, caso contrário não há suspense.
At the 2013 Comic-Con Vince Gilligan explained exactly how he and the writers imagined Walt got the poison to Brock, and it wasn’t, as I had thought, through Saul. Instead they pictured Walt as the “Evil Juice Box Man” going into Brock’s school and giving him a juice box that had juice from the poison berries. And if you’ll look closely at episode 413 “Face Off” (thanks to Greg below for pointing this out), when Walt busts into Saul’s office, Francesca is shredding school schedules. It’s a big spreadsheet of classes and times but there are notes on the side that seem to be one student’s personal schedule, most likely Brock’s. So, kudos to the writers on including that little detail, and again to Greg for noticing that what’s was being shredded. End of update.