Any time I have received a plush toy in a sealed plastic bag (which is generally how they will ship), the bag itself has been labelled with instructions to shake the toy after removing it from the bag to "fluff it up" and "make it look its best on the shelf." As you only specified "letting it sit," try giving it some really good shakes to see if that helps.
Dirty stagnant water can be a breeding ground for mosquitos, harmful algae, molds, etc. It is very important to drain the kiddie pool completely after every use. Allow it to fully dry under the heat to get rid of any moisture left. For cleaning, you can use bleach or vinegar (diluted in water) to disinfect it. I would suggest leaving the solution in the pool ...
Stagnant water and sunlight are the perfect recipe for green algae. Ask any aquarist about it.
The best way is to cover the pool carefully, so no dirt, leaves, garbage, insects... can get inside while it is not used. Ideally, the cover is totally non-transparent - to avoid the green algae.
Regarding substances, anything has the potential to damage the area ...
You can do a web search for "pipe cleaner craft". These wires have a fuzzy coating and are intended for crafting.
They can be found very cheap for hundreds of foot-long wires of various colors. For example, on Amazon.com I see 600 pieces for about $15 US.
A mild bleach solution with a toothbrush or a toss into the washer with some clothing while in a bag works for me. I have also tried washing them with the water hose, this high powered water works as a pretty effective cleaner. But you should always soak them before washing as it increases the effectiveness of the wash, as most dirt is now loose....
Wash them. The easiest method is probably to put them in a bucket in the sink, then run the water over them for a minute. Take the bucket out, and dump them on a towel, and let them dry.
Boom - no more dust!
Simply soak your models in a sink of 104ºF/40ºC water with a bit of detergent. You can put in a little bit of mild bleach if you want to disinfect them. You may want to gently scrub them with a paintbrush.
Once you've left them for a bit, rinse them off with clean water and leave them to air dry.
Advice taken from lego.com.
As someone who spends far too much time every summer in maintaining an above-ground pool, take my advice.
For a small pool like this, just empty it. You can use the water for watering your plants.
If you have left it for a few days, clean it with a brush and washing-up liquid to remove the algae and all the other nasty stuff that will be present. Rinse it ...
The correct way to do this is to treat the pool as a swimming pool:
Buy a pool filter. You can get cheap portable pool filters.
Use chlorine, and check the water quality regularly. But if you do this, you can't just overturn the pool to empty it into your yard as the chlorine will damage the plants. You have to empty the pool into the sewer.
Any chemical ...