Translation:How many children do they have?
Well, 几 is used when the answer implies a number less than or about 10. Usually people have less than 10 children, so in this case we use 几. But if we ask about time, day of the month, month or year, we use 几 too. I can say that 多少 is used in questions about something material, and the answer to this question may be more than 10.
:-D you mixed up a few things there. 1. The one child law has been (recently) replaced by a two child law. This means that you can have up to two kids of any gender. If you have more you face fines and risk loosing your job. 2. It was never about the gender. Though I heard that in the rural areas boys are still preferred over girls, this is a preference of the people and not the politicians/law. This had/has a lot of implications which you probably have heard about. 3. "How many kids do they have?" could refer to more than one couple and even for one couple the answer might vary even if the maximum number of children was 1 - so it remains a valid question.