When Polonius says goodbye to Laertes, preparing to send his son off to the European continent, he quickly transitions from the formal "you" to the informal "thou." This seems to indicate a closeness and a lack of distance between them. Polonius can be casual with his son. He tells Laertes,
There, my blessing with thee.
And these few precepts in thy memory
Look thou character.
(act 1, scene 3, lines 62–64)
He proceeds to give Laertes a slew of advice about various things, indicating that—though he clearly worries about his son, as a father would—he does ultimately trust his son to make good choices.
After Laertes leaves, Polonius turns his attention toward his daughter, Ophelia. With her, he uses only the formal "you," implying that there is a more significant emotional distance between them. He does not feel as close to her, as familiar with her, as he does with Laertes. Rather than give her advice, he gives her directions, commanding her to think of herself as "a baby" and to "tender [her]self more dearly" and keep a distance from Prince Hamlet so as not to make a fool of Polonius, ostensibly by calling into question her reputation as a virtuous maid. When she explains her behavior or tries to tell him about Hamlet's professions of love to her, Polonius scoffs at her and even insults her, saying,
Affection, puh! You speak like a green girl
Unsifted in such perilous circumstance.
(act 1, scene 3, lines 110–111)
He does not trust Ophelia as he trusts Laertes, and he seems to have no faith whatsoever in her judgment.