The pink rug here is nice. It has gradients and tone-on-tone polka dots. http://www.younghouselove.com/2013/01/rolling-out-the-pink-carpet/

And, at that price, you won't weep when your 8-year-old daughter decides she can no longer tolerate pink and blue is now her forever favorite color.

Speaking from experience on the pink issue. :) Love it while she does.

I bought a Carpel braided chenile rug at LL Bean for my little one's nursery. You can also order the rugs directly at Carpel's website. It has purple, light green, pink, yellow, and cream. It is super soft for her to crawl around it, and it's two sided so you can flip it if you don't want to clean it frequently. Also, the rugs are made in the U.S.

http://www.capelrugs.com/cutting-garden-blue-bell-rugs

Good luck with your search!

I could see this one with your (great) bed cover :http://www.designersguild.com/usa-shop-online/shop-home/home-accessories/area-rugs/eberson-grass-area-rug/#.UTXq_TBWySo

The green grass carpet would be my choice too, it's kind of neutral - vs pink- and green is a shooting colour I would welcome in a kid's room

I would go with a solid or an understated pattern. I think a flokati style in grey or white, or a (faux) sheepskin would look soft and yummy with that sweet, sweet bedding. What about these ideas?
http://www.rugsusa.com/rugsusa/rugs/rugs-usa-greek-flokati/natural-grey/200FS09-305.html (comes in a few colors)
Grey on grey pattern: http://www.rugsusa.com/rugsusa/rugs/rugs-usa-modern-damask/grey/200FIOCO05-508.html
If you want more pattern: http://www.rugsusa.com/rugsusa/rugs/chandra-rugs-far6208/grey/217FAR6208-79079R.html

We actually skipped the rug altogether and put this down in our daughter's room when she was crawling (and then walking and falling down a lot). She had a large bedroom with hardwood floors, so this worked well for us. We went with the "brights" color scheme because we had a lot of different colors in the room. http://www.skiphop.com/product/playspot.html

Also, I did a lot of research before purchasing this. It's one of the only ones available in the US that does not leech toxic chemicals, so, added bonus.

You should check out our website, NIBA Rug Collections, we curated a collection of children's rugs for this exact reason. With so many rugs catered to children's rooms in a tasteful, whimsical style - you can't go wrong! http://nibarugs.com/subcategory_ind.php?id=59

I would get a cotton rag rug. They come in various colors and sizes. You get a solid color - there are many that go with your bedding - but it has the advantage of having variegated colors in that one color in it. Yet the somewhat solid color doesn't clash with other stuff in the room, and doesn't limit the color and pattern you can use everywhere else in the room. They also come in patterns if you prefer that.

The reason I recommend this over the other rugs shown is that I would want a rug my baby crawled and played on all the time to be easily cleanable. Lets face it, it is going to have all sorts of stuff leaked on it that you will want to wash out. I have found that I can wash a 6' x 9' one easily in a triple-size washer found in laundromats and apartment building laundry rooms. And dry in the dryers there. Used one in different rooms for decades, including my kitchen. Always came out really clean. Wore like iron. Could take your child through an entire childhood and into their first apartment.

Love those foam tiles, too. Even more easily cleanable!

I have a cotton rag rug that was in my brother's room when we were kids - probably 30 years old. You can imagine how grotty it had got in 30 years! When I was re-decorating my study in white and green, I chucked it in the washing machine with several packs of green dye - result was a fabulous green rag rug with all different shades of green. If you want something hard wearing but more a single colour than your typical rag rug, I can heartily recommend dying them - works great.