relative utility..

दश कूप समा वापी दश वापी समो ह्रद :|

दशह्रद सम : पुत्रो दशपुत्रसमो द्रुम :||

dasha koopa samaa vaapee dasha vaapee samo hradaH

dsash hradasamaH putro dashaputra samo drumaH..

How can ten children be equal to a tree?Any mythological story related?

( The simple meaning of the sloka is 

A pond is equal to ten wells

A lake is equal to ten ponds

A son is equal to ten lakes

and 

A tree is equal to ten sons.)

My answer is

It is only a matter of relative utility.. 

according to the relative extent of help the various objects can provide..

Koopam.. is a well...

a pit of very moderate width but deep in size, usually dug on the earth by human endeavour.. which contains water which could be drawn again by human effort.

Vaapee... is a Pond...

a larger collection of water.. usually formed by nature by filling wider deep areas on the face of the earth with water.. There might be some natural springs of water too.. (Vaapa means gentle growthand build up) Here water can be drawn easily from the surface.. and the source would serve larger number of people..

Hrada is a Lake..

a far more huge collection of water.. formed by nature.. filling large terrain of comparatively deeper parts of the earth's surface with water.. 

In addition to providing water for all without much effort, the lake can be the source for rivers and rivulets.. 

In some works, the sea itself is viewed as a huge lake..

Lakes like the ones in America or the Manasa Saras in the Himalayas mean a lot to the earth..

Puthra.. is the Son.. 

Son is called puthra because every person, by his activities in life becomes liable to be sent to a hell called Puth पुत्, and the son, who serves the parents and performs the death rites is expected to save them from this hell..

Even otherwise a son is expected to be of great help and assistance to the parents in their sunset years..

Druma --the tree

The tree.. is called Druma 

especially because with long and widely spread branches with deep foliage, occupies huge areas scorched by the sun and provides comparative shades and coolness for all things below.. 

They drink water from deep inside the earth, absorb stale air from the atmosphere and produce fresh oxygen, food,.. 

They shelter birds and small animals.. 

They provide timber, fuel,, They give us many things, and even sacrifice themselves completely to ensure our welfare..hardly expecting or getting anything in return..

So a Pond is ten times more useful than a well.. A lake is ten times useful that a pond.. A son is ten times more useful than a lake and a tree is ten times more useful than a son or is equal to ten sons..

Here practical wisdom speaks and I really do not think any puranic or any story is involved.. It does not seem necessary..

And how can a tree be equal to ten sons..?

the question is left to the imagination of the readers..

Of course, Wells, Ponds, Lakes and Trees are all really useful..

I am not sure about the place of a Son in the chain .. 

Too many sons break the law.. and become counter productive in real life,, especially when the parents are stepping into their old age..