Admins will face issue of Child picklists not being displayed for selection while synchronizing the Picklists from one instance to another instance using Instance Synchronizing tool.
Successfacrtors BizX instance
This is an Expected Behavior of the product.
We do not display child picklists on the instance sync page. They will be implicitly synchronized during parent picklist synchronization.
KBA , LOD-SF-PLT-PCK , Picklist Management Issue , How To