Make sure your wife has time for herself; with two children it is easy to feel that a woman's life is being lived for other people (and so a husband's legitimate requests for sex just feel like one more obligation). Help with the children/housework, pay for someone to do the housework, and/or babysit while she takes a craft class or engages in a hobby. Once she feels she's had some time for her, her sex drive may increase (and of course, set the right mood).
