In Breaking Bad Saul has been shown at Andrea's new house, where he delivers Jesse's money directly to Andrea. Both her and Brock are familiar with him, we even see him conversing with Brock.
Also later in the 5th season, Walt randomly shows up on their door-step, I don't recall Jesse ever telling him where they live...it connects the dots a little, it would also explain how Brock was poisoned.
Saul gave him Lily of the Valley
The doctors and Jesse believed that Brock came across the flower growing naturally and ate some poisonous berries.
And how often would a kid question someone they knew? All Saul would have to do was hold out his hand and watch him eat the poison. It parallels nicely too, with Jesse's all too foreboding words "you're just feeding them hamburgers, just like every time. Also note that Saul is distant with Walt throughout the 5th season, and uncomfortable whenever asked to perform a task for him.
So did Saul poison Brock?