I think if you choose the smaller roads rather than the motorway route to Porvoo, that might be slightly more interesting, but probably not significantly more interesting and obviously the smaller roads have lower speed limits which mean the travel time is longer. And yes, the historical centre of Porvoo may not be that interesting for the younger visitors. Traditional point of interest in Porvoo for kids is Brunberg factory outlet, which sells variety of sweets. But that is slightly off from the city centre and I doubt cruise lines offer tours that include stop at that candy store.
And if you want to spoil kids with products that can harm ones teeth, there is one alternative closer to Helsinki, Fazer Experience in Vantaa offers also brief exhibition tours with a chance to taste some of the products made there.