Question:

My child urinated by mistake in our swimming pool.  Will chlorine be sufficient to purify it? 

Answer:

In the Name of Allah, the Most Gracious, the Most Merciful.

As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.

In principle, there are two types of swimming pools in terms of Fiqhi applications and rulings: 
1)     A large swimming pool
2)     A small swimming pool

Large swimming pool:

A rectangular pool which has a surface area of 20.9 square metres or more will be regarded as a large swimming pool.  Likewise, a circular swimming pool with a diameter of 5.16 metres or more will be classed as a large swimming pool.[1]
A large swimming pool will be deemed pure regardless if some impurity has breached it.  There is no need to further clean such a swimming pool.  One may continue swimming and using the swimming pool.[2]
A large swimming pool will only become impure when an impurities colour, smell or taste becomes dominant in the water.[3]
 
Small swimming pool:

A rectangular pool which has a surface area less than 20.9 square metres will be deemed as a small swimming pool.  Likewise, a circular swimming pool with a diameter smaller than 5.16 metres will be regarded as a small swimming pool.
A small swimming pool will become impure by any impurity falling into it, whether it is visible or not.[4]
The method of purifying the pool would be to add pure water into the pool until the water begins to overflow. Once the water begins to overflow the pool would be pure.[5]

Chlorine alone cannot ritually purify a swimming pool which is regarded as small.[6]
 
And Allah Ta’ālā Knows Best
Mufti Faraz Adam,
www.darulfiqh.com