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We have a kiddie pool in the backyard that the kids enjoy playing in during the summer.
My family tends to forget to drain the pool, so water may stay in it a few days... or a week... too long.
Is there something that I can add to the water that will keep the bottom of the pool from getting scummy, but won’t kill the grass or be dangerous for young children?
(A couple of drops of bleach? Vinegar?)
Example photo — our pool is a Vinyl-type blow-up pool, not a plastic shell. 
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 where the water falls when the kids play. Some bleach will work, some baking soda might work (I did not use it personally for this purpose, only for other hacks).
The easiest is to empty the pool after each use, and fill it again when needed - provided it is not too big.
If you want to hack it the complicated way (ahem... the high-tech way), use a system made of a water pump and a mechanical dense filter. While the system works, use a cloth or a sponge to mechanically clean the pool. Leave the system work until the water becomes clear again - a sign that the debris is now inside the filter.
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 to allow scum to breakdown then use a gentle scrub to buff the surface with soap to remove any scum left. Do the same to the exterior of the pool. After cleaning, let it dry completely. It is important to clean kiddie pools regularly because kids tend to accidentally drink water from pools etc.
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 a few times to remove all of the detergent.
MOVE IT - to a new location. If it has only been covering a patch of grass for a few days, the grass will recover pretty quickly. I now have a permanent spot for the pool, but before I did this, I used to have TWO pools and keep moving the empty one around the garden and pumping the water from one to the other.
Refill it with lovely fresh, clean water. This is FAR easier than using chemicals, pumps, filters and covers to keep it clean. Take it from me. None of that stuff is cheap, if you calculate how much it will cost you to refill it every day for a whole season, my guess is you won't save any money by treating it like a proper pool.
This may not be a very PC thing to say but it may also be worthwhile just throwing away the pool at the end of the season rather than clean it. You will never get it as clean as a new one and you will spend time, energy, water and detergent trying to achieve it. They are not very expensive so it is likely to be more sensible to get a new one, if, indeed, your kids are still interested next year.
Final tip: if you do throw it away, consider buying the next one during the off-season when they are cheaper.
The correct way to do this is to treat the pool as a swimming pool:
Any chemical you put into the pool to impede algae growth will be herbicidal.
