Sharing data betweeen Multiple Databases
Don't think there is any existing solution for this.

Though it should be possible by building an extension on your main site and pushing the data to the child sites everytime an entity is created/updated.

For eg, Create separate groups for each child sites, eg ChildA, ChildB and ChildC.

while creating a contact if Group A is selected, this contact will be pushed to the first child site.

We have done similar implementation for one of our client but it needs some pre-requisite information to be stored in the ext files, eg site key, API key, etc.

If interested to check our ext, get in touch at jitendra@fuzion.co.nz.