Step 2: Design it
I also opted to have a good low BB so that it would be easy to put a foot down to stabilise when stopping.
For a front wheel I went with a 20" wheel because I had plenty of them spare and plenty of good strong forks to go with it. But more importantly a small front wheel helps keep the cargo space big without the wheelbase getting too big which is a pain for manouvering in tight spaces.
I went with a fairly long front end, and in hindsight I could have shaved at least 3-4" (75-100mm) off the front end length for my immediate purposes
My design was chosen to be simple and easy to make. I also wanted to leave the design open to adaption later as my children grow. When they outgrow the box I can convert this into a "Child in-front Tandem".
By using a single rectangular tube all the way from the bottom bracket to the front headtube everything would be fairly "self jigging" and solid. I think the main tube is 50mm wide 100mm high and 3mm wall thickness and 6082 grade which isnt spectacularly good stuff, but cheap, weldable and easy to find.
If you dont have thousands of dollars wort... sorry cost of Solidworks, a pencil and paper will work just fine too, it is just a bit harder to tweak it about until you like it.
The central circle is where the kinkstand will swing if I ever get round to making it. The aim of the kickstand is to allow the children to climb in and out on their own while keeping the wheels on the ground so it doesnt tip around the stand.