Suspect I understand your question all too well. This has been my life for many years, working from home with kids. As work from home gurus Paul & Sarah Edwards wrote, it does get better once they're past 6 years old, but the basic issues remain the same - too little time, too many interruptions, too many other priorities that crowd out your own.
So, yes, as wonderful as GTD and other time management strategies may be, if you can't reliably schedule your time and stick to it, then you need a whole different approach. What I've learned is, the two key elements are flexibility and energy management (as opposed to time management).
Look for ways to add flexibility to your routine, so you don't HAVE to do something today, but OTOH are ready to take advantage of little bits of time, etc., when they do arise. For instance, stock up on those things you're always running out of, carry your reading with you so when you're waiting you can at least do some. Have a reliable calendar system so you don't go nuts keeping track of everybody's movements (this gets really tough once they're teens). There are a lot of ways to add flexibility, once you're alert to it.
But energy management is pretty much the holy grail, and more & more it's being recognized that time management can only happen for folks who are first healthy & energetic. So Job One is taking care of yourself - eating, exercise, sleep, taking time to recharge, etc. Many CEOs realize this, and so should anyone else concerned about productivity. Basic health is key. One of the best books on this is the classic, 'The Power of Full Engagement':
Once health is established, there are many more energy-management skills that need to be acquired. Energy 'debt' is way more costly than money debt. A lot of us Type-A achievers have a hard time with this. We want to do everything ASAP, stick-to-the-plan, make progress, BUT when you keep overdrawing your energy 'bank account', very bad things start happening - starting with doing dumb things.
One interesting point made in that book is how most all the big industrial accidents occurred late at night. Of course, one of the main things they're looking at with Saturday's Asian Flight 214 crash is how it came at the end of a very long day (Shanghai-Seoul-San Francisco). We all know this CAN happen but don't think we'll do something so dumb, but it isn't just big mistakes - unrested, even our routine productivity will go way down. It's much better to invest time getting some rest on a regular basis.
Learn the fine arts of letting things slide and of doing things not-so-perfectly, so you can have time to rest. Learn to take a nap, a valuable skill I didn't acquire till past 30. Learn to manage email in 10-15 minute bursts. These are just a few examples of the sorts of skills needed for productivity in 'real-world' conditions ;-)