As @Kal_Lam correctly states, presently the _id field is a Kobo backend assigned field, and one which - since it isnt part of the actual ‘parent’ form definition itself - isn’t included in the shared xml-external data file.
It conceivably could be included, but that would be a new feature change. Please feel to propose something in the Suggestion Box, explaining what you want and why you think it’ll be useful to others. 
That said, you might be able to accomplish something similar or equivalent today, albeit without the actual _id field per se. All form submission already come with an associated UUID, which uniquely identify that submission. Parent form submissions included… Typically the submission UUID iis hidden from the user, as metadata, but it is still part of the submission XML, so in practice you can [sometimes] access this UUID from within your form. And by basically duplicating this UUID into one of your own form fields (read-only of course, since you dont want the user changing it) then your own uuid copy will appear in the parent form as a shareable field to your child forms. So you could possibly achieve what you desire using this uuid instead of the _id.
Have a play around with these parent & child forms, and see if it might accomplish what you are thinking. Note, you need to add a name = 'data' to your parent form settings to fix the XML root instance identifier (which is otherwise a random string in Kobo) so you can correctly reference the instanceID element in the metadata XML calculation; ie /data/meta/instanceID. This instanceID element contains the generated UUID for your submission; it is always prefixed with ‘uuid:…’, hence my substring-after() to strip it off.
Parent.xlsx (18.3 KB)
Child.xlsx (15.5 KB)
Once you have this UUID in your Child form you can possibly use it as an equivalent unique identifier for the Parent.
Caveat
I say sometimes above because there appears to be an issue in Enketo where the calculation copying the metadata instanceID isnt firing (!?), so parent form submissions submitted from Enketo end up with an empty uuid field. 
However, when submitted from KoboCollect, the uuid field is correctly populated with the instanceID.
The Parent form and Child form have both been tested and work fine on KoboCollect. The Child form has been tested and works fine on both KoboCollect and Enketo. The Parent form does not work correctly on Enketo (as described above).
I’m investigating why Enketo doesn’t (it may be a bug to fix…)
