Framebuilders - Is this material appropriate

Think of making a bike "for the fun of it" for my 11 year old daughter who has hit a growth spurt. She would like to try a kids triathelon. Can I use cm 1inch tubing in .035 inside diameter for a bike for her? Will this tubing fit inside lugs? I appreciate any help!

Standard size road lugs are designed for 1" top tube, 1-1/8" seattube and a 1-1/4" downtube and headtube. http://www.cycle-frames.com/bicycle-frame-tubing/LUGS/
Strait gauge .035 thickness CrMo (or even hi-tensile) would be sufficiently stong for the brazed ends of the tubes but will make a very heavy and stiff frame for an 11 year old as compared to using butted tubing.

I would really suggest you start with reading the Talbot Manual before trying to re-invent the wheel (or frame).
http://bhovey.com/Masi/Scans/Talbot/index.htm#1

My thinking was to try this to see how it goes before building another one. As someone suggested, I can get cromo from aircraft spruce for cheap in case I mess something up. But the 1 inch .035 said it only weighted .3 per foot. Is that heavy? I thought it would be pretty light? I even thought abotu for going lugs and directly brazing the fish mouthed tubee togther.

I have read some of those manuals by the way.

My thinking was to try this to see how it goes before building another one. As someone suggested, I can get cromo from aircraft spruce for cheap in case I mess something up. But the 1 inch .035 said it only weighted .3 per foot. Is that heavy? I thought it would be pretty light? I even thought abotu for going lugs and directly brazing the fish mouthed tubee togther.

I have read some of those manuals by the way.
Anything you do for an 11 year old is probably going to be overbuilt and consequently it will also be heavier than need be, but probably lighter than any storebought frame that fits.
Depending on what size frame your are building be mindfull with your shopping if you do go butted because you may end up hacking the butts off to get the proper lengths if you are not carefull.

That said Nova and Bikelugs both have quite cheap tubes sets or tubes available. Ceeway also sells his complete straight gauge kit which will have everything you need.

fillet braze and you open up lots of possibilities..

What do you mean?

What do you mean?
Fillet brazing involves carefully mitering the tubes for a close fit and then brazing them together without lugs. What fietsbob means is that tube angles are not limited to the lug angles when the joint is fillet brazed. The downside of fillet brazing is that the technique requires lots of practice to get a strong, nicely contoured fillet.

Example of fillet brazed joints by framebuilder Dave Kirk:

http://i32.photobucket.com/albums/d7/k4drd/Bicycles/Frame%20Tube%20Joining/DaveKirkFilletBrazedHeadTube.jpg

Here are head tube to top tube joints using brazed lugs, TIG welding, and fillet brazing:

http://i32.photobucket.com/albums/d7/k4drd/Bicycles/Frame%20Tube%20Joining/TubingJointTypes.jpg

I read you cant braze 4130 because brass causes cracks in the weld? I guess all brazing rod does not have brass?

I've seen that too, but millions of bikes beg to disagree. I don't think the source of that is credible. IIRC, Jesse James' welding book? I like that book, but I don't rely on it for technical information.

Hundreds of thousands of fillet brazed Schwinn Superiors, Sport Tourers, and Super Sports (http://www.sheldonbrown.com/schwinn-braze.html) were built using 4130 tubing and don't suffer from cracks.