This is a follow-up question to Why did Aunt Petunia mistreat Harry Potter given that Harry was so rich and she had more to gain by treating a rich boy well?
One reason Aunt Petunia mistreated Harry was that she did not know he was rich. If she knew, she would naturally have treated this golden goose nicely. Her retirement would be secured if Harry financially supported her in old age.
Why didn't Harry tell Aunt Petunia he was rich? Surely every boy wants to be treated nicely by his family.