Can't help but wonder what their new employee will be like. I won't be surprised if they're secretly a spy sent to observe Millie. The only question is whether they're working for the grandpa, the priest, or the prince.
I saw it as more metaphorical than explodey type of bomb. As in, her magic power as a kid is big, as she grows up it will be even bigger. Somebody with that much magic won't be able to hide it forever, and then bad guys will try to take advantage of her.
It's not exactly wrong to think of it as the explodey type of bomb, too. A five-year-old losing control of their magic and accidentally flattening several adults is definitely concerning. Without formal magic training to get her powers firmly under control, there's no telling what might trigger another disaster, and assuming little kids are expected to get stronger as they grow up, it wouldn't be farfetched to imagine Millie accidentally demolishing a building, potentially maiming or killing everyone inside, including herself.