I'm a 37 year old female, and I've been married 15 years with one six year old child. I feel for you...believe it or not, my husband is the one who won't initiate sex, but that's a topic for another day!
Anyway...I'm not sure where you live, but a woman named Laura Corn has some books out that have little sealed pages in them--half for the man and half for the woman. You rip out a page and unseal it, and inside is a little game or fantasy that you have to initiate with your partner. Some of them take some planning, and some are more spur of the moment. Go on Amazon.com and look her up--she has several books. Sounds to me like if your wife really likes sex, she may just not feel comfortable being blatent about it, like, "Hey, you wanna do it???" Perhaps the anticipation of planning out a little "adventure" with you would at least give her some ideas to start with. You could take turns and do one every weekend or something like that.
And, just a reminder, the next time she does initiate sex, don't hold back on how much it excites you! I can only speak for myself, but one of the best parts of sex for me is seeing how much I'm pleasing him. So a little positive reinforcement might just carry over to the next time too! And I don't mean saying, "See honey, I TOLD YOU I like you to initiate sex"....just let your responses speak for themselves. :-)