Haven't they grown

All Beth has to do is drive her son to his Under-14s away match, watch him play, and bring him home. Just because she knows her ex-best friend lives near the football ground, that doesn't mean she has to drive past her house. But she can't resist. She parks outside the open gates, watches from across the road as Flora and her children Thomas and Emily step out of the car. Except? There's something terribly wrong. It's the children. Twelve years ago, Thomas and Emily were five and three years old. Today, they look precisely as they did then. They haven't changed at all. They are no taller, no older... Why haven't they grown?