I’m currently building up a 20″ wheel full sus Scott Spark 20 Junior for my oldest (a tall 6yrs old).
Scott do make a 24″ wheeled version but it’s more XC than DH and will probably be too small for your lad.
I can’t offer much advice on frames / bikes but I’ve learnt with this build:
– Some parts for kids bikes can end up costing a fair bit because you can’t tap the vast second hand parts market e.g. shorter cranks, smaller chain rings
– For my next build I’ll definitely look for a frame that uses a standard rear shock e.g. RP23. I’ve found getting a replacement for a non standard one really time consuming and it would have been too expensive if I didn’t have some help getting one back from the USA
– A complete overhaul of a tired second hand bike could end up costing more than you thought as all of the little extras are added up e.g. modifying front forks, wheel build costs (probably less of an issue in your case), adapters / bolts that are needed, new bearings, maybe new tools. A full powder coat will be £50 – £70.
– Sourcing the right second hand parts is time consuming (both time spent looking, and a long elapsed time is needed as they don’t come onto the market that often). If you’re looking at 26″ wheels that will be a big help as decent 20″ or 24″ tyres, inner tubes, rims, etc are all harder to find and / or expensive.
I’m enjoying the process of learning and sourcing the parts, and I’m looking forward to the build, but it’s taken a lot longer than planned, and it’s definitely cost more than I thought.
I echo the points about weight. Most standard kids bikes are really heavy and the bike weight is a huge % of their body weight. A custom build with carefully selected second hand parts will be a lot lighter …this was my main reason for finding a tired / cheap second hand bike and doing a custom build.
Eg. The carbon bars I’ll be using are over 300g lighter than the stock bars, the cheap KCNC stem will save nearly 100g, the Schwalbe tyres and Intense racing light inner tubes will save 732g over the stock parts.
Hopefully that will help the thinking process.