Be child’s play
Definition 1 of 1
Pro Tip 1/3Simplicity Implied
Use when describing tasks that are so easy they require minimal effort or skill. 
Learning to ride a bike was child's play for her.
Pro Tip 2/3Subjective Ease
Remember, what is child's play for one person may be difficult for another. 
He's a chess master, so winning that game was child's play.
Pro Tip 3/3Informal Tone
It's an informal expression, suitable in casual conversations but not for formal writing. 
Building the bookshelf was child's play, unlike assembling the wardrobe.