I searched and I could not find a step-by-step process that described how to setup a Parent Child configuration using Differencing disks in Hyper-V. I was first taught about this trick in the Connectix / Virtual PC days. The VMWare parlance is linked clones, I believe. Anyways, I started a TechNet Wiki article a couple of days ago and just finished it today. When you get a chance, please, check it out http://social.technet.microsoft.com/wiki/contents/articles/hyper-v-virtual-machine-vm-parent-child-configuration-using-differencing-disks.aspx. This is a Wiki, so you can always help to make the article better. These directions are primarily for saving hard disk space and time when you are building a test lab.
I already put this in the article, but it bears repeating: One important item to note is that when you configure a parent virtual hard disk for a “child” differencing disk, you should never start or change that parent vhd. If you do, you will effectively lose the “differences” that are marked in the differencing disk. To be safe, you need to configure the parent vhd to be read-only so it is not mistakenly changed and you’ll want to make a backup copy of it somewhere in case the file becomes damaged or corrupted in some way. /