The subplot of Polonius' relationship with his children highlights the lack of genuine family relationships at the Danish court. As with everything else at Elsinore, power, not love, is what determines relations between Polonius and his children. We can never know for sure if Polonius does indeed love his children. If so, he has a funny way of showing it.
Once he's bid farewell to Laertes, who's off to Paris for his studies, Polonius immediately instructs one of his lackeys to go with his son to France, where he is to spy on him. Once there, he's to put it about that Laertes is involved in a debauched lifestyle. How people react to such news will determine whether or not Laertes really has been a naughty boy. Most normal parents would balk at such ruthless manipulation of their children's lives. But not Polonius. As a hardened veteran of the Danish court, such power moves are second nature to him.
Even worse is his manipulation of Ophelia, whom he shamefully uses as a pawn in an elaborate game to determine the reasons behind Hamlet's unusual behavior. Ophelia is emotionally destroyed by the experience, yet Polonius doesn't seem to care. As always, he's only interested in the political consequences of Hamlet's vicious rant against his daughter.
Once again, we see how proximity to power distorts family relationships, which, as with all royal courts, are determined by power rather than by bonds of love and affection.