About six months ago, I came across the OpenRC community, which is centered around the idea of open source R/C projects, and was started by a gentleman named Daniel Norée.
If you jump on Thingiverse (or even Google for that matter) and search for 'OpenRC', you'll see a lot of neat open source projects with build instructions and STL files for printing.
One project in particular that caught my eye was the OpenRC tractor created by a gentleman at makitpro.com.
I thought this was going to be a fairly quick and easy project, but it took about 5 months for me to complete. Before you freak out and steer clear of the project, I put the project on hold for a while once I realized that it wasn't going to be a quick and easy project.
Also being newer to the R/C world, I wasn't sure what parts I could swap out with others, especially since it looked like a lot of the 3D components were designed with a specific part in mind.
The parts list for the project referenced a lot of parts on Banggood, so that's where I ordered most of my stuff. Ordering overseas (and not paying extra for expedited shipping) added weeks to the project, as I spent most of my time just waiting.
Aside from that and a number of failed prints (this was the first project I ever did that involved printing with flex filament, so there was a bit of learning curve), it was pretty fun.
I didn't record much of the build process, but the video above shows the final product.
If you have some spare time and dollars, I think this is a fun one that you and/or your kids might just enjoy. There are even attachments you can print and build to extend the project and continue the fun.