I don't see any problem. We actually rode around some when we got tired. The lower level is inside the bus and then they have an upper level that is open. On a nice day, it's really enjoyable so your 5 year old would probably like riding in the open air! There are earplugs you can plug in and as you go by certain sites, it will tell you about them. Kind of hard to understand sometimes but other times you can hear just fine. Now the buses can get crowded but you can always wait for the next one and hope it's less crowded. We really enjoyed using the bus.
We wanted to see Montesserat this time, but don't think it is going to work out. Our plane arrives around 10:30 am on Friday before the cruise and by the time we get checked into our hotel, don't think we will have much time to see Montesserat. We're right off Las Ramblas, so we are just going to take it easy and walk around (and of course, we'll be checking out El Corte Ingles!). One thing your 5 yr old might enjoy is the Magic Fountain Show (I think that is what is called). They have it on Friday night from 9:00 pm - 11:00 pm. We talked with a young married couple last year who thoroughly enjoyed it, so we decided we would do it this time. Suppose to be a beautiful lighted water fountain display.
Hope this helps!!!