E. coli) spread easily in untreated water. (According to the CDC, germ-killing pool disinfectants like chlorine and bromine are unsafe for kiddie pool use because it can be hard to gauge how much disinfectant should be added as water sloshes from the pool throughout the day.)
Dumping the water from your pool each day can also help mitigate any potentially disastrous accidents. The CDC mentions that some state or local laws in your area may require fences or barriers around inflatable and plastic kiddie pools and slides. And the US Consumer Product Safety Commission recommends that you either fence, cover, or empty your portable pool (PDF) to ensure your child’s safety.