How hard is this tour for 5 to 8 year old children.

My 7 year olds though it was fun and not at all challenging, but I think a 4 and 5 year old may tire out before making it to the end.

Not too bad depending on their personality, my 4 year old wanted to be carried a lot of the time. However, when my son was that age he wanted to walk the whole thing himself and he did it pretty much with ease.