If they charge you more it's a premium seat. If people consider it a more desirable seat it's a premium seat. But this distracts from the question, why they wanted someone to give up such a seat rather than anyone, so they can be together.
And yes it is reasonable that parents should be able to sit with their young child. If the system doesn't do it automatically, human decency is that the steward(ess) should politely ask people, and someone should offer. In a polite world, the steward(ess) would thank them and give them a free drink if it's not free to start with.
It's like pausing to hold the door for someone with a baby in their arms... you're not obliged to do it but it's still the thing to do.
On our recent long-haul flight, for some reason we couldn't check in to the long 2nd leg of our flight until we got to the airport from our first flight... so the plane was absolutely full and we were all placed in middle seats. My wife is a very nervous flyer who couldn't cope sat on her own for 8 hours so we got someone to switch.