I know this is an old post but thinking about this actually added some immersion to Hearthfire for me so I thought that I should post my answer here, since it helped, if that makes sense.
Well first of all, let me just say that I could not for the life of me figure out what my main characters daughter, Lucia, would grow up to be - that is until she got smacked into the sky by a giant passing through my Lakeview Manor. I was able to get my characters other child, Blaise, out of the way in time but not her.
In game, she disappeared for about 2 or 3 days then appeared back in her room one night. I came up for a good way to explain this in character, which is that Lucia was injured but found a way to survive. She healed her wounds with some plants in the area, and then made her way back home, hiding the whole time so as not to be attacked. So I figure that would make her a good alchemist and rogue. Not to mention that whenever my main plays hide and seek with her kids, Lucia seems to disappear - and she can't be found until the game ends. The fact that she can hide from my character, the Listener and leader of the Thieves Guild who has a fairly high sneak level herself, says something.
As for my characters son Blaise, well he's not to bright - in the aforementioned games of hide and seek he always runs smack into his mother, so he'd suck at stealth - and also the fact that he threw tantrums until he got his own wooden sword and practices on the dummy in the cellar constantly, like literally from morning til night if his mom lets him - he would be a warrior, probably using either sword and board or a one handed sword alone.
So yeah, basically to sum it up, Blaise would be a warrior, possibly a Companion like his mother, and Lucia would be a rogue or spy of some sort who is a master alchemist.