Easy mod designer for kids?

Hello,
my boys are 10 and 12 and Beamng fans. In the school, together with the engineering school, a large project on topics such as statics and design is carried out once a year. First the group designs it, and then builds it out of strips of wood and paper. At least test it with a weight load.
The next project is a ski jump. Then we came up with the idea to build the design in Beam NG like a simple mod and to test whether the statics are good. E.g. with wind and vehicles.

But we've never changed a mod, don't even know how to do it. So far only modeled in landscapes. The object consists only of wooden bars and surfaces, what do we need to learn how to do that?

Thanks Andreas

Attached is a picture from 2020, where my son took 2nd place, A roof for a stadium.

 

If you wish to simulate stress forces of object, then you need to build object as a vehicle, to get basic idea of how structures are made in BeamNG, look this link:
https://wiki.beamng.com/JBeam_Introduction

From this page you can find information how to start building your structure:
https://wiki.beamng.com/Vehicle_Creation_Guide

This is tutorial how to make a vehicle:
https://wiki.beamng.com/Introduction_to_Vehicle_Creation

All in all, there is no single answer, but lot of resources where you need to build understanding of whole process:
https://wiki.beamng.com/Vehicle_Creation_Megapage
https://beamng.com/threads/tutorials-index-list-of-useful-guides.51454/

Overall, static objects are hugely easier to start with, but those don't react to forces, hence the name static.

You could also open some of the ingame props and examine how they are made, like barrel for example.

In game content is in content folder of BeamNG.drive_install_directory\content\vehicles\ and vehicles contains cars, but also barrels and cones in zip file that you could extract and start looking into how they are made.