Answer:
The smallest possible length is 0.83m
Step-by-step explanation:
Given

Required
The smallest length of the tank
Since the tank is cubical, then the volume is:

This gives:

Express as 


Take cube roots of both sides

Rewrite as:

Approximate

If the ratio is 14 childen to every 5 teachers, and there are thirty teachers... there are 84 children. hope I could help!
Can you post the scale drawing please so I can help you