Hi,
you can manage parent-child relations in few different ways in sebod:
a)
using fieldX or groupX - here you have just one record in the database
table (e.g. orders) and all data is stored to parent (in json format,
child field is added using field X or (more likely applicable to your
case) using groupX, where you can add a whole group of fields from child
content type (see
http://www.seblod.com/resources/extensions/plug-ins/group-x-field for details)
b)
using some field that will relate the two content types - you can use
Joomla article field or select dynamic/checkbox dynamic (with query that
selects articles from some category). But in this case you need to
store this 2 article separately e.g. first creates order, then when user
saves the article, redirect to the child form.
With some ajax
javascript you could potentially store both at the same time, but this
would be a custom solution outside the scope of this forum.
Alternatively you can also use server side (php) code to do the same,
e-g- to get category id you would first need to store it or run a query
to get next insert id from category table.