Hi

Depending on size of kids I would evaluate the longtail track in the cargo bike jungle. Probably the most agile alternative.
Maybe the yet to be seen (?) foldable longtail, which should be easier to carry onboard a train?
...and building a longtail might be a fair amount of welding as a start up project. But I must warn you; This is addictive so there is a risk that you will conitinue building... :-D

//hp