I suspect the problem is one of people mis-understanding the complex rules for claiming dependents. In decades past, the term "child" was actually allowed by tax software approved for IRS filing, but presumably too many people thought that meant any child they happened to have some financial relationship to, rather than the very specific relations required by tax law. By requiring more specific terms, it made it harder for people to mis-claim children as dependents.

For the purpose of filling in the tax form.