Late afternoon was perfect in early May but I have no idea of the temperature in June. The whole kayak trip was pleasant and not strenuous.

Am not sure what that price would be but i e mailed them.a few times and they got back to me really quuckly so try that.

Yes its a 2 hour tour but i think they offer different ones

I can't speak to the issue of how to actually buy a ticket for a child. One possibility is that the cost is the same for everyone, regardless of age, hence the one button. But, in terms of the activity itself, I see no reason why an 11 year old would not be able to do this, especially if they're already familiar with how to kayak. The activity is limited primarily to the side canals, so it's a slower pace and you don't get the wakes that you would get on the wider canals. The pace is slow, as the guide takes the time to explain some of the history, architecture and/or culture of the different areas you paddle thru. For those who have not kayaked before, that's not an issue either as they do a mini-tutorial before you even get into the water and allow newbies an opportunity to get the hang of it before advancing out of the canal from where you launch. From my experience with this activity, I don't see anything that would raise flag for an 11 YO being able to participate.