You need to 'date' your wife again! Bring her home flowers, make her feel special and loved.. Trust me, she is NOT going to have sex with you if you demand it from her!
Each of you need to really listen to one another. Put yourself in her shoes, just like she has to put herself in your shoes. You love her, she's the mother of your children - You married because ?? Why not list the reasons why you married and what you love on her. Instead of looking at the negatives, look at the positives.
Get a babysitter or take the kids to their grandparents house for a weekend so you two can go out to dinner, spend time alone. Couples NEED to have that alone time as husband and wife, friends - Not just live day in and day out routines. Work, home, dinner, cleaning up, dealing with the kids, then sleep.
Hope this helps.