I did a build up of the Gable car J-567 (Clark Gable never actually owned the cat but used it extenively) about 30 years ago. It came out well though much of the filler has shrunk. Keep in mind all I had at the time was good old green stuff. The way you describe doing your work is exactly what I did years ago. I look forward to seeing your progress, even if you abandon the project. This is coming from a guy who took 15 years to build a Judkins Victoria! Best of luck.
I would encourage you to check out the current photos of the Cooper car. If you PM your address I can get a very good article about the two short wheelbase Duesenbergs, AKA (but not from the factory ) the SSJ cars.
Once upon a time I used black spark plug wire (1:24 scale of course ) for fender welt on a Cord Sportsman I was building. I think it came out looking spot on. Maybe consider that? The Phaeton is looking excellent.
It is so exciting to see kids getting into the hobby. I'm hoping that I can get my girls started. I think I started when I was about 7 but what do you think is a good age for kids to begin building? You have done well getting them involved and their workmanship is excellent!
I have had the privilege to drive many Classics including 9 (I think ) Duesenbergs. They are indeed very fast and really live up to their reputation. I have built this model several times over the years and it is my favorite kit. My words of advice pertinent to building it relate to the brake backing plates. They are quite fiddly and care must be taken with alignment both vertically and horizontally. I know some people have built jigs for this purpose but I'm not that advanced. Have fun with this one! There's lots of reference material out there just under Google images and the Auburn Cord Duesenberg Club website is great also.
I think I have driven 9 Duesenbergs over the years with SJN 564 being the best. For obvious reasons Duesenbergs are my favorite Classic to drive but I have to tell you that a V12 powered Packard is quite a thrill too, as is a Franklin, particularly with 12 cylinder power. Lincoln K's and V16 Cadillacs are just trucks. Beautiful trucks, bur trucks all the same. A blower Bentley is pretty freaking amazing too. I have sampled just about everything out there and when tuned correctly a Full Classic is a thing of beauty on the road just as they are on a Museum floor. And of interest to readers here, while complicated, Rolls Phantoms are really great cars to drive.