child's play

from The American Heritage® Dictionary of the English Language, 4th Edition

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

from Wiktionary, Creative Commons Attribution/Share-Alike License

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

from the GNU version of the Collaborative International Dictionary of English

• n. light work; a trifling contest.

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

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