Police said 4 boys fell to the water while 6 of them perched on a steep of the tank and attempted to wash their hands.
Residents have rescued two of them but the two victims drowned.
The deceased were identified as residents of Kumakulam and Ukkakulam in Vavuniya.
Meanwhile, a 35-year old man who had allegedly killed his father was arrested by Pannala police.
The victim had been murdered with a sharp object inside his own house at Udugama in Pannala yesterday.
Police said an altercation erupted between the two when the suspect demanded the ownership of the land which had led to the murder.