Besides the dashboard route (which is the easiest and a good one if linking data is the only requirement) there are two other options, neither of which is “simple” but may make sense:
You can gather the children from doctype A and populate children in doctype B onload. This can be done in a custom script if that is a requirement, but honestly, I think it’s a little messy.
If your requirement is view-only and you would send the user to doctype A to edit the data anyway, an HTML table showing the information could also work. This is not stateful data -
render_template only knows what you tell it - so this solution has flaws too.
Synchronizing the data across two views would almost definitely require server side code to push the changes back and forth. I’d say there would have to be a powerful use case to make this worth the effort but it could be done.