Let's say you have a parent domain:

parent.com

and children subdomains under that parent domain:

child1.com

child2.com

child3.com

Is there a way to use just the following within parent.com:

User-agent: *
Disallow: /

Considering each child has their own robots.txt stating:

User-agent: *
Allow: /

Or is the parent robots.txt still going to have to make an exception for every single subdomain:

User-agent: *
Disallow: /
Allow: /child1/
Allow: /child2/
Allow: /child3/

Obviously this is important and tricky territory SEO wise so I'm looking to learn the definitive and safe, best practice method here to sharpen my skills.

Thanks, Bryan