Q1 : For the most part : no. All of the SEO principles are the same no matter what the website/technology. You want to optimise the on-page factors of your html, title tags and internal linking. This is achieved with careful selection of your skin, careful editing of your pages : all of which are parent/child indepedent.
The one gotcha with child portals in DNN is the use of the ?alias=childportalname Url that the framework automatically reverts to. This isn't very good for SEO because it uses a 302 redirect away from the site root when you request it. (ie mysite.com/child is forwarded to mysite.com?alias=child)
Q2: To successfully migrate your ranking you wil need to either set up 301 redirects for all your .htm pages to the equivalent dnn .aspx pages OR you will have to setup your DNN site to .htm as an extension, and create your DNN site to match your old site structure, so that each ranked Url will match a DNN page.
You need to do this quickly before search engines hit 404 on your old site urls and decide to remove them from the index.
To achieve Q2, the module that I distribute will assist you in doing this. You can't do this with the standard DNN provider. As a bonus, it will also clean up the urls for your child portal so that you don't get the ?alias=childname redirect.
The module is called the iFinity Url Master. It's a total replacement for the DNN Url rewriting module.