Dark energy builds up inside a magical child when they suppress their powers, creating an Obscurus that can violently burst out of them (WFT).
References from the canon
- Credence Barebone was often punished with a belt by his mother Mary Lou, who knew he had magical lineage. Eventually this built up the Dark energy and turned him into a dangerous Obscurial full of pent-up anger and frustration (WFT).
Magic Type Dark Magic