As with all things - different brands size things differently. E.g. a Marks and Spencers size 10 is different to a Next size 10 - we all have experience of knowing what suits our personally needs. I personally need a size bigger at Next!
The general rule about children's shoes is that they should support / aid walking - not create additional obstacles to overcome (like flip flops encourage a child to claw their toes to keep them on - which may cause probelms later in life). If you stick to shoes that fasten on (buckle, lace, velcro) and that if they lace up that the there is a gap between the uppers when fastened to ensure that they are functional (not just a style).
As brands vary in their sizing and you do not have the services of a shoe fitter that you trust - if you draw around your childs foot when they are standing (so that the foot is at it most extended) you can cut out a strip from the longest part as a guide. The general rule is that you should have a 1cm gap between the end of you guide and the end of the shoe for it to allow adequate growing room, as well as accommodating the natural lengthening of the foot during walking. As to the width - it is more a question of how much space your childs foot needs and this is down to genetics, and I think this is a common sense issue. This is applicable to adults buying shoes as well.
As to Clarks et al - we must remember that they are a profit making company and any technological gimmick can give the impression of expertise, and allows them to charge more for their products, theoretically because we are buying a service not just a pair pf shoes. I would suggest that the expertise of the person using the technology should be questioned. A teenager working a saturday job is not an expert in my opinion!
I also think that off the shelf products are OK - if you are being a consciensious parent and regularly checking that shoes are fitting properly. There is a lot of manufacture knowledge out there and even the cheapest of trainers (bought from a market stall) has a lot of the fancy technology that goes into a £100 pair of trainers these days. We just need to be wary of the guilt complex when it comes to children - expensive is not always better, just often a well marketed brand.
Sorry to rant on - but have a real issue with the theory that shoes cause foot problems. Genetically bad feet (family trends)create foot problems when put in shoes that do not accommodate them and when we wear shoes that create changes to the foot posture for long periods of time (high heels). As with all things common sense rules - wear the right shoes for the right job. I personally wouldn't go shopping for a whole day in a pair of high heels!