Tom is white, and Helen is black. They have a black daughter and
a white son. Is it true that in real life, when a white man marries a black woman, they have a black daughter and a white son?

The DNA lake is a complicated thing

LEt's go with the Base that the man is white and the woman is black, and that they have a son and daughter for purposes here only. Based on genetic variables, these are the possibilites:

D=White
S=White

D=Black
S=Black

D=Black
S=White

D=White
S=Black

D=Mixed (A Tan-ish color)
S=White

D=Mixed
S=Black

D=White
S=Mixed

D=Black
S=Mixed

D=Mixed
S=Mixed

Of course, you've also got to factor in the rare chance of albinoism