My 4 year old daughter woke me up the other day saying "Wake up you've got to work. That's what grown ups do".
Having an office outside the house is well worth the expense; it makes it the distinction between "work" and "home" very clear to everybody, and when I am in either place, I am able to give my full attention to the task at hand, be it work or family.
By the way: you might want to ask your 4-year-old for some details concerning what she thinks "work" is all about. When asked what job she wants to have when she grows up, mine says "Tinkerbell."
Part of the problem is that, if you're anything like me, computer time isn't all work time. Sometimes for instance, my 2 year old and I look at videos of animals on youtube or talk to grandma and grandpa on skype, and at 2, she's just beginning to acquire words like 'work', let alone fully understand the implications.
The biggest issue for working from home is meetings. You can't have people into your "office" and be taken seriously, and there are just some things you can't do at Starbucks.
At least, that's my experience. YMMV.
That, or they're a perfectly normal young child.
And it works both ways: when you hid your kid crying (which you'll hear even with the door closed), it overrides pretty much everything else in your head.
But it is hard. It's impossible to explain to your parents that it's not ok to ask you to "just drive your brother somewhere, it will only take 10 minutes". From their point of view, it's entirely reasonable. From your point of view, it's a waste of maybe an hour, considering switching costs and everything else.