Update Feb 2nd 2015
Fixed the issue of Sims putting plates and books into the crib. Included warning in catalog description for the safety of your RL babies.

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All items of this set are matress-hight and intersectable, so they can be placed on the crib without "moveObjects", and they won't hinder Sims from using the crib.

The Blankets

I think the reason why there is no custom cover blanket out there (that I know of), is that the child is sitting waist-deep in it, when awake. I don't have the ultimate solution to this either. The best thing I could think of was to enable live-dragging. So you can just drag the blanket off the child in livemode when it wakes up. Of course the blanket will stick out of the crib then. As I said, it's not the ultimate solution.
Please note that although the blanket is live-draggable, it is NOT storeable in inventory.

There is one version for toddlers and one for babies, each opening on the correct side, being in the correct position and shaped to aviod clipping. Like my sledge, it works better with the unwrapped baby. The wrapped baby cuts through it's blanket slightly, when breathing.



The Pillows

The pillow is not draggable, because it comes with lots of slots for additional cuteness. Depending on what you put there, and in which order, you might not be able to use them all at once, but anyway they are meant for choice, not for suffocating your little ones (see warning above ) I found that sometimes it's easier to place the deco first, and then move the whole construction into the crib. The M key is really useful here. Please also keep in mind that there is still the teddy-slot attached to the crib.
Again there are two versions for babies and toddlers. It was not just a matter of turning it 180 degree, as I optimistically imagined...

Technical

All four objects are found in buymode under:
Sort by Room/Nursery/Furniture
Sort by Function/Kids/Furniture

Name	Price	Channels
Blanket Baby	20	1
Blanket Toddler	20	1
Pillow Baby	10	2
Pillow Toddler	10	2

You can see the presets and channels on the picture attached. I want to add that what looks like distorted texture on that image is actually the drapery of the blanket.


Screenshot Credits: Everything on the baby as usual by the wonderful QuizicalGin.
Mouse: Free Store Set Harvest Bounty

Polygon Counts:
Baby Blanket: 140
Toddler Blanket: 140
Baby Pillow: 98
Toddler Pillow: 98

Additional Credits:
Thanks to the creators of Sims3PE&OC, Milkshape, Blender, Gimp, and their various plugins.