This issue has been discussed. Past threads may be of assistance. Do a search on the forum's search tabs.
Repeating what I mentioned before and above, several programs are family friendly. Others may look at you with disdain knowing that you have responsibilities outside of work. Why? They know they might not be able to get as much work out of you. It's a combination of them wanting to get work out of residents, an old-school mentality that residency is about suffering, and a worry that at the worst moment where you're needed, you may have to leave work because your kid needs to go to the doctor.
A good program will be family friendly. Hardly any program will reveal themselves to not be family friendly because it's not PC. It's a glass ceiling. Some workplaces don't want to hire the guy with the long hair, but will not openly tell the person he was not hired because of it. They'll just send a nice letter saying something to the effect of "thanks" without mention of the hair--but the hair was the real reason.
For that reason, some applicants I've noticed just openly state what's going on because they'd rather get rejected from such a program and they'll be steered towards the family-friendly programs---and that's what they want. That's great and the ideal situation if you are a single parent. Unfortunately, not everyone can be in that position. Some people are in the position where they will not have many choices and the revelation that they are a single parent to a program may further cut down the choices available.