We've got the regular and the slim airics. Slim airic would be good for going to festivals without a car as more portable but still comfortable. Regular airics are very comfortable for camping - and I am rather curvy, a tad overweight and have back problems. My DH uses the thermarest version of the Wee Airic for mountain walking/backpacking and finds that very comfortable.
I have two fat airics and they are amazing, but was thinking of getting a smaller one for my kid. Whats the best one for an eight year old boy, one with a bit of growing room. The fat airics are looooong!
I've just looked at the Alpkit website and all Airics seem to be out of stock. I think regular and slim are the same thickness - but the slim version is obviously narrower (around 12.5 cm narrower), so not so good if you keep turning in the night! The Fat Airic is 22 cm longer than the slim and regular Airics. For an 8 year old boy a 180 cm long mat would be long at present(presuming he would be 120 cm - 130 cm tall), but will see him through his teenage years.
Have you tried Decathlon - they have a good range of mats including some that pack down really small. They have them out for demonstrating too so you can lie down and test them! Might at least do for the children.
If it helps I have a Thermarest allegedly 3.8cm thick, and find it really comfortable. It folds down the length before rolling so makes a tiny package - like a fat kitchen roll. (27x15cm and 900g). A Quechua A200 light is probably the equivalent.