This really could be a number of things, but women tend to seek emotional support before sex and men are just the opposite (normally), so this creates a real challenge in relationships.
Possible causes could be everything from her just being exhausted to what heartbrokencanadian said - she's getting her emotional needs filled by the kids.
It's really important the the two of you get to a point where you can openly share your feelings without blaming one another - even if you need to bring in a counselor.
You said yourself that you 'complain' about your sex life. That's sure to be a turn off to her. Rather than complaining start to find ways to connect with her. If you can connect with her on an emotional level, your chances of improving your sex life go up greatly!
As a woman, I have a feeling that sex has become just another to-do item on her list and she has nothing left to give. You see, women tend to give and give and give... And then all of a sudden we have nothing left to give and we don't even know what happened.
One thing that's worked really well for Dan and I is to take some me time - an hour per day is ideal. So what exactly is me time??? Just that - no kids, no husband, no chores, no demands. Give her an hour per day to do something for her that she really enjoys - you should do this for yourself as well.
Also, men are far more easy to please sexually... Have you ever heard of the three before me rule? That's means she get 3 orgasms before you even think about it. While this is a bit extreme in my opinion, you get the idea - How about one?
If she's not enjoying sex, truly enjoying it, then again, it starts to feel like yet another thing she's giving to the relationship...
Do you see how things can get out of balance?
Your goal now has to be to bring things back into balance. Give her the me time that we talked about. Start to romance her like you did when you first met - you may have to win her heart all over again, but I hope it's worth the effort to you. Take her on a date - every week! A real date - Go where she wants to go. No questions asked. She needs to see how hard you a really trying (not complaining). Sweep her off her feet. you've done it before. You can do it again!