I've no idea why you'd want to edit afterwards when it's not hard to get it right in the first place. If I were you I'd google "portrait photographer" and look at the portfolios of people who are already selling. See the ones whose style you like and try to work out why you like them and what they've done to achieve the end result.

If you're concentrating on children I'd say work with a longer focal length and just shoot as they play. Use a large aperture to spearate them from the background. Don't try following them, you'll only get the backs of their heads, just wait for the opportunity to come to you. Most of your shots fail because the kids aren't in a 'kiddy' environment. Standing next to a brick wall would work well for a moody teenager, but children work best when being shot playing. Take them down the park, shoot them jumping in puddles, that kind of thing.

Finally, learn to walk before you try to run. Portraiture is one of the hardest types of photography to learn. Follow the advice given above and post some on the Appraisal board for crits. That's the best way to learn.