fennesz i disagree with your big bike summary with regards to spesh hotrocks. They are indeed min scale versions of the bigger bikes.
My son has developed from 12 to 16 and will get a 20″ in december. All have been lightweight when compared to any other bike i have looked at, they have all had near perfect service and all have a rear coaster brake which has suited him fine until now. He is naturally ready to do away with the coaster brake and i have looked at the 20 model and would be surprised if the Isla bike is much different in weight.
That said i am going to have a better look at the isla bike as they do look good quality, however, as my lbs is a spesh dealer i will find it difficult to go past their excellent service. My son is the only 5-6yrs old i know of to wear out a coaster brake. Spesh had to strip one from one of their stock bikes as they dont even carry them as parts.
Oh and finally, i recommended the 12″ Hotrock to the sister in law for their little lad. I got a thank you the other day when they £99 bike 2 yrs ago sold for £65 on ebay. Thats exceptional value