A pool with worms is just normal. Please do not get scared. Still for the kids you should take care. Worms appear in the pools and it is seasonal and they breed in great number. Just clean the pool removing the existing water as quick as possible. Clean the pool, its bed and surrounding with bleaching power and allow fresh water into the pool. Take care while allowing fresh water. Just observe minutely if the larvie are still present. Yes, it is time-consuming. It happens.