Not every child actor turns into a trainwreck, there are probably more who turned out good than bad, but people like to focus on the negative. Christina Ricci, Chlomo, Natalie Portman, the Fanning sisters, G Hannelius, Hillary Duff, etc... all turned out well, as well as Zendaya who was Bella's co-star.
Millie's career is going very well and I imagine her going more the Zendaya route than Bella. But that's just my opinion.