After I returned from covering Vietnam, I worked for newspapers and went to school and raised my daughter. I had to take her to some of my shoots, even though she was a toddler, but being was able to have some 'help' from the sheriff's department or police department while I did my thing.
She was with me while, as she got a bit older, covering the more interesting civil rights, antiwar coverage of the early 70's, and the counter music scene, too.
When I went to editorial she was helping to build sets and did voice over work. She doesn't care about the work now, but that's fine with me.
I couldn't afford child care before she was old enough to go to school, and after school started for her, either, so she was with me. Her mother ultimately got custody, and I went back to active duty.
You can make it work, as I look back, with eyes shaped by the fullness of time, rather easily, considering the alternatives.
If you are going to be a freelancer, you'll likely need to be one of two things, a very good specialist or a very good, broad generalist. I don't know which path you'll chose to take, but you'll need to focus on that portfolio. Your children don't have to be a liability, and you shouldn't ever think of as such. You might even toy - for a short time - as a children's photographer. (My first regular paycheck came that way, with a 5x7 camera inside a furry stuffed bunny). It's rewarding work (in more than one way - a professional tip for everyone who doesn't known, parents think their children are beautiful, don't mess up the shoot and you more than half way home).
Ansel Adams told me that your in your work you need to see the light in and sense its mood and make sure your exposure captures 'that'. Sort of short, but kind of encapsulates the Zone system he was fond of, and really shortened the process, too. All of it should be a fairly quick-ish process (back to the 'how long' part of the question.
Working for the press, some of our setups were 2-5 seconds long. The burning building is falling right now, take the picture. In children's photography, the expression of the day is at 'this' moment an passes in an instant. But the same is true for the great outdoors guys, too. The alpine glow only lasts a few seconds, and you haul butt to get there to find the right place to setup, and move the tripod a few times and adjust it to make sure you get it right and likely, if you're lucky, you hit mark only a couple of times.
A lot of people think that good photography is just a matter of luck and gear, and I think that maybe if they understood what they were saying, I'd agree. If you're smart and experienced to pick out the right gear to use for the right situation and you've spent enough time with the gear and trained yourself on it and the craft, you'll be able to catch the 'lucky breaks'. ;-)
So take the kids, enjoy them and shoot them with your camera and make them a part of your adventure and scenery. They can help, in many ways develop your portfolio. This won't be the only time you'll have to work with things getting in the way in getting your shot.
One more, and the most important thing, as has been already pointed out, enjoy your children most.
My best, and I hoped it helped.