Asking about children is definitely a valid question. No one takes more time off work than someone with a young child either in day care, or school. If they are in a position where attendance is important, I can see why an employer would ask about this.
Co-workers I've been around with children are constantly out of the office due to "my child was sick", "the school called about my child", "it's a school holiday", "my kid hit another kid at daycare", etc etc etc. All this is above and beyond the personal time taken for illness and holidays.