If you want to create a DNS domain, you can do so from dnsmgmt.msc. Simply create a new zon and give it the child name at creation, like child.example.com
Keep in mind, in Microsoft AD/DNS terms, sub-domain is different than a child-domain. If you have the domain blah.com, you can go into the primary zone and just right-click and create a sub-domain folder with the name of subdomain. You then technically have subdomain.blah.com. All it is, is a folder.
Now a child domain requires a new DC which will be a seperate authoritative server for its DNS namespace. That is of course if you use it as a DNS server as well. You can actually still use blah.com DNS server to host your child DNS namespace. You will need to run DCPROMO on a new DC and go through the options to create a new domain in an already existing tree. Since it's a child domain, it will use the same namespace as its parent, thus why you add it to an existing tree.
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http://technet2.microsoft.com/WindowsServer/en/library/e3f241b5-82a0-4c24-a56a-bfc00ce1b5c21033.mspx?mfr=true
If you want to create a DNS domain, you can do so from dnsmgmt.msc. Simply create a new zon and give it the child name at creation, like child.example.com
Now a child domain requires a new DC which will be a seperate authoritative server for its DNS namespace. That is of course if you use it as a DNS server as well. You can actually still use blah.com DNS server to host your child DNS namespace. You will need to run DCPROMO on a new DC and go through the options to create a new domain in an already existing tree. Since it's a child domain, it will use the same namespace as its parent, thus why you add it to an existing tree.