Who controls a 529 plan — the parent or the child?

The account owner controls the 529, not the beneficiary — and that's usually a parent or grandparent. As owner, you decide how the money is invested, when and how it's withdrawn, and you can even change the beneficiary or take the money back (paying tax and the 10% penalty on earnings if it's non-qualified). The child named as beneficiary has no legal right to the funds and can't redirect them. This is a major contrast with UTMA/UGMA custodial accounts, where the child gains full control at the age of majority. The owner's control means a 529 stays a parent asset for financial-aid purposes, which is favorable. You can also name a successor owner so the account passes to someone you choose if you die. Retaining ownership keeps you in the driver's seat throughout.