Generally, much of the sightseeing in Europe involves history, which 7 year olds may get bored with quickly. The ships will priovide babysitting onboard if he/she gets tired of sightseeing.
If the 9 year old enjoys history then I think that is fantastic, I hope he enjoys it.
In truth, I generally would not recommend taking children to Europe until they are old enough to understand complex topics like government, money, fine art, etc. That is what will be on YOUR mind and I hope the children will not be too much of a distraction if they are not being entertained.
I have seen kids in Europe, and they often looked bored. I have also noticed that towards the end of the cruise they are spending more time on the ship "with grandma" or whoever else may be along who does not feel like touring every day.