Since I have a 5 year old, in my personal opinion, the 12 hour day would be too long on her and would wear both of us out if I had to hear her complain about it. It is a physically tiring day since you do have a lot of time of the bus in between stops but it is long. For that reason, I think the 9 year old might handle it better.
Also consider the 9 year old can climb the leaning tower of Pisa (super cool!) but the 5 year old can't.