I feel your pain. I went through this same issue not too long ago. And while the advice PowerDNN gave me wasn't what I wanted to hear it was accurate advice.

I determined that a lot depends on the TYPE of child portals you indend. I.e. What the end users are going to be doing with those portals. If they are going to be installing new modules or even just doing a lot of "tweaking" then child portals is probably not a good way to go since a problem in just one of the child portals will effect the entire installation including the main portal.

With regards to scaling - if you were to have one child portal that goes off the charts with traffic you would need to seperate that portal into it's own install / server. From what I understand it is possible to seperate a child portal but it is definitely not easy or very "clean".

On the other hand if your child portal users are just going to be using a pre-built structure - not adding modules, pages, etc. and you don't expect the traffic of any one portal to be that great (now or in the future) then child portals can save you money in module licenses as well as save you time and headaches in portal maintenance and upgrades.

On the project that I was working on we elected to go with seperate installs.  Our project was a business app for each of our clients and we didn't want to take the chance that a problem in one portal would crash all of our customer's portals.  Although if I had it to do over again I might look harder at using child portals as these apps were pre-built with the end users only doing data entry. The other issue that is involved is server resources required and performance.  I would hope that using child portals would consume less server resources.  With regards to site speed and performace I would GUESS that the seperate install approach would be more efficiant.

please do me a favour to avoid any confusion for newbees, and use correct terminologie:

• multiportal feature of DotNetNuke allows to have one installation with multiple portals - this is the subject of this thread
• each of a portal can be either a parent or child portal, where Childportals require a parent and reside (virtually) in a subfolder of it.
  E.g. Parent: www.mydomain.com, child: www.mdomain.com/child
• do not mix this up with DNN installations in virtual directories, which may have the same path for parent portals as web site child portals - but is separate DNN installations: www.mydomain.com/dotnetnuke (which also may have childportals www.mydomain.com/dotnetnuke/child)
• and last, not least, IIS Web sites may use sub domains: dotnetnuke.mydomain.com

Thanks to all for contributing.