child's play

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Something very easy to do.
• noun A trivial matter.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Used other than as an idiom: the play of a child.
• noun idiomatic Something particularly simple or easy.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun activity by children that is guided more by imagination than by fixed rules
• noun any undertaking that is easy to do