While you can enable multi-org to connect multiple business units to the same Org, this is not supported and not recommended. Support will advise against this and can be problematic when synchronizing a large number of records, as multiple concurrent requests will be made on the same object from different BUs, which places a large (and unnecessary) load on the Object. Also, bear in mind that once you implement multi-org, this is irreversible as it changes the underlying structure of your account.
As you can't share Synchronized Data Extensions, you are correct in that you will need to write query activities to update DEs in your Child BU. FYI (and for the information of others who want to do this), you can retrieve Synchronized DEs directly from a Query Activity in the Child BU using the ENT. prefix (e.g. use
However, you can't do this for accounts that were created after January 2018, as they have implemented a measure to prevent this (due to a GDPR policy update). But you can open a Support request asking them to enable this again for you.