My son is 7 and racing NBL in GA he is pretty good and he rides a Mongoose proJR i want to make it lighter and him faster the bike is pretty light, want a new fork and crank any suggest! Im a Father that supports all extreme sports Radicalrus
For a child racing I would suggest some Answer Alumlite Mini fork if you want lightweight and money is not an issue. Also a Chris King headset would help make the front end feathery as well!! Not to mention Crupi makes a mini stem that weighs almost nothing. One more thing S&M makes some one piece aluminum bars that will no doubt compliment all of those other high end parts, and they come in several sizes so you can find the perfect rise for your son!
For cranks for a smaller child Answer or Crupi arms will do perfectly and if your kids bike is like almost every other JR bike I have seen a Action-Tec european bottom bracket will make for the lightest crankset you will ever hold. With either of those crank arms you will use a 110mm chainring so I can guarantee you will be able to find the perfect gear ratio almost anywhere. You can also polish off this combo with more titanium parts, did I forget to mention the suggested BB is titanium, if you use some titanium chainring bolts. You can never get your kids started with clips too early either, and some Eggbeaters would make this drivetrain even lighter!
Lastly for wheels I suggest some Chris King cassette hubs (or some Crupi low flange hubs if you prefer a cheaper super light weight non cassette hub, however the engagment of the king hub will make you and your son fall in love with them) laced to a Sun M19A rim with titanium spokes and aluminum nipples, granted that rim IS a sew up, so you'll probably have to get a shop to install the tires, the weight saved IS worthwhile!!!
I raced for several years and did mechanic work for alot of that time, take the suggestions for what you want but I'm telling you those are the parts I saw consistent winners riding time after time!!