Read Galatians 4:1 Using Other Translations
Now I say, That the heir, as long as he is a child, differeth nothing from a servant, though he be lord of all;
I mean that the heir, as long as he is a child, is no different from a slave, though he is the owner of everything,
Think of it this way. If a father dies and leaves an inheritance for his young children, those children are not much better off than slaves until they grow up, even though they actually own everything their father had.