I've been tossing this idea around in my head since visiting my family recently. My 9 year old sister likes to where dresses over jeans. I thought it was kinda cute on the kid, so I was wondering if it were possible to re-categorize an outfit(the dress) to the tops category so that my sim kids can wear jeans under their dresses?

I'd like to do it myself, I'm trying to learn since it's becoming increasingly more difficult to find people willing to take the time to create/edit items I'd like to have.

I just don't want to do something stupid and have my game explode on me :/ I know there are probably custom dresses out there that are in the tops category, but I'd still like to know if it's possible and how to do it so that I can move any dresses I'd like to move over to tops

Yes, it is possible. You may have to modify the dress though to scale the legs of the mesh so it is not visible though. Just make sure you don't delete anything though because it will mess up the morphs (learned this the hard way).

Possible yes. Anubis has made a few items like this, check his blog. Also, it would be easier to make a shirt into a dress for this than to go the other way around. The tricky part is not the meshing, its the UV map.

Thank you

You might need to remap it cause the pants texture will likely go over top of the dress

Like Robo mentioned, you most likly need to remap the mesh and redo the textures.
This is alot of work....