Most 5-6 year olds are racing on bikes that weigh about 15 pounds. 12 is considered light. I got my son's FMF micro mini down to 11. But that was still 25% of his weight. Considering my road bike is less than 7% of my body weight, that makes his bike-to-rider weight ratio terrible. I'm hoping to get below the 10 pound range for him and still have a bike durable enough to make it through the season.

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there has been plenty of Ti bmx frames come and go over the years. More suited to a mini or micro mini.

always have been to flexy for pro size bmxers.

I guess so then.

you can pay <$400 USD for a ~ 1.5 alu or ~ 2kg cro-mo pro size frame these days.

what would a Ti bmx race frame be? and approx how light would it be in order to be stiff and durable enough?

First to answer your actual question. Carbon bmx frames.

Mongoose made the first that I know of, if you find one stay away they were extremely fragile.

Currently you have GT with the UB2. MBK from France makes a full carbon bmx and personally I think it's one of the coolest bmx frames to ever come to market. However neither of these come in 24.

A year or two ago I saw a really, really nice custom that Serotta did, it was carbon tubes with ti lugs. Imagine an ottrott in a bmx frame. I imagine if you could get them to build you one you'd be looking at a $5000 or more price tag.

Go back a couple of years and you'll find a mix of carbon tubed alloy lugged frames. Both FMF and Avent made them in cruiser. The Avent is not too difficult to find in expert cruiser which is a kids cruiser, but is difficult in pro cruiser.(I know because I'm still looking for one to match the micro my daughter has.) The FMF isn't too difficult to find. These look really good, but honestly aren't any lighter than the light aluminum frames out there.

A bmx ww project is fun, I've done it a few times for myself and my daughter. The problem is as was mentioned, bmx parts are built for abuse. I'm lucky in that I'm light (130 lbs) and after 20+ years of bmx I'm smooth and not rough on my bikes(relatively) That's allowed me to get away with mixing stuff from outside the bmx world. Places to save weight are things like the seat/post/hubs/spokes/cranks/seat collars etc... anything that you can pull from the road or mtb realm. Take for example the Redline you mentioned. While the flight cranks are light for bmx they are still almost 1200 grams. Replace it with a Storck Isis mtb crank, a TA ring, a Ti ISIS bb and a bb adapter and you save about 1.5 pounds, a little over 600 grams. Several changes like that and you can easily knock 3-5 pounds off a stock bike, but you quickly add thousands to the price. You can now get carbon rims from companies like warp9 as well. It's possible to build a ww bmx bike but it can be as expensive as doing a road project. My advice would be to get something like the stock Redline, or buy a used supercross or something similar and ride it for a year. See how you do on it and and even if it's something you want to stick with. Then you have a nice foundation to start a project from.