If you really want to get into this....there are a few things you can do!
You could file a complaint with Paypal if you paid using that method -- it will certainly force her into action if you feel it is important enough. It will also anger her b/c Paypal will suspend the money in her account -- but she has to respond either way. I did this once, but it was a $140 auction (not diapers), so it was worth it to cause a ruckus.
There may be a way to report her to ebay for fraudulent/misleading auctions, too.
In terms of feedback: I would leave honest feedback about her and rate it as neutral or negative if you don't get the response you want when this is all said and done. Be calm and objective when you write it. Also, if you have been a member for a while and she hasn't, your feedback tends to mean more than hers anyway to savvy ebayers. She shouldn't leave retaliatory feedback for you - you paid for the auction in a timely manner and in the way in which she requested. So, you deserve a positive as a buyer.
I would wait to leave feedback. Sellers tend to get pretty peeved if buyers leave a negative too quickly. Send her another email with some specific options for a response (like, a refund, the soakers, or a partical refund) and some way to save face (like, "maybe you didn't know that the diapers were supposed to come with soakers..." blah blah) -- and if she doesn't respond, then start escalating. Tell her again before you escalate, too. I told my seller that if she did not email me with a UPS tracking # by the end of the business day that I would be reporting her to paypal and to ebay....and sure enough, I received a NEXT DAY AIR tracking number. Voila!
Good luck. Ebay is a real pain.