There are several long threads about the game's system of genetics in the forums for PC/Mac versions, for Virtual Villagers as well as Virtual Families as they use the same system.

There are less blond head choices than there are brunettes or redheads so blonds will be rarer. There is also a random element, so you might just be unlucky. Although it is not exact, you are most likely to have a blond child if at least one parent is blond. It's more complicated than that, but that gives a rough answer.