Child’s play you say

An article is written about a tip for designing a web page. I came across a phrase they said 

"Child's play you say".

I got no idea what it really means. The given context is as follows:

How floats behave

Nothing too complex, but still pretty cool right? Child’s play you say.

 Ok, well before we get into the part where floats usher in a world of bacon-loving unicorns, 

let’s back up for a second to talk about what’s actually happening here. 

In the web world, our HTML is bound by some rules, in particular, the normal flow.

Could anyone please kindly shed some light on me?