Wally West is Iris' nephew, and in the comics, he becomes Kid Flash after surviving an accident that's essentially the same as the one that imbued Barry Allen with his speedy powers. Eventually, Wally went on to take on the mantle of The Flash after DC's Crisis on Infinite Earths shook up their comic universe. Lonsdale has been cast as a series regular, but there's no word at this time whether we'll see him suit up immediately or if he'll inevitably don a costume farther down the line.
Flash Executive Producer Andrew Kreisberg said of the casting, “Just like when we met Grant [Gustin] for the first time, we instantly knew Keiynan embodied all the heart and courage of a hero. We are so excited to be bringing this much-beloved character onto the show.”
I know nothing about Lonsdale's work as an actor (I haven't seen any of the Divergent movies), but the producers seem to be pretty good at casting, so we're hoping he does a great job with the role. The Flash revs back up with its second season on October 6th.