I don't have children, so never had to deal with that myself, but I do know people who went through grad school with children. It does make it harder, because you're juggling more things. One thing grad students with children seemed to have in common were VERY supportive spouses or other relatives as caretakers (one grad student's mother moved into the town after his son was born, and made it possible for him to be away from home so much).
Some of the grad students I knew with children had older children, so it was a bit easier...the kids were old enough to understand why mommy wasn't home. Others had very young children, and really did a lot of juggling. They couldn't count on being able to get work done when at home like the rest of us did, so did have to stay in the office more when they needed to read or write. When they were actively doing research, it was a lot of night hours, after the kids were in bed. One person would come in after her kids were off to school or daycare, take her own classes, get home in time for the kids to get home from school, supervise their homework, make them dinner, get them tucked into bed, then head back to the lab to do her research.
I wouldn't recommend it if you can postpone having kids until after grad school (post-doc years are a lot easier time for having children, in my opinion), but if you already have them, don't let it hold you back from grad school as long as you realize it's going to be a little more difficult juggling everything.