This is one of the best psalms to teach a kid, I think. I often recite it to younger son along with Isaiah chapter 40 and Psalm 104. With a tweaking, any kid could understand it. Of course there is the little hurdle there with verses 19-22 and I suspect there's a time when one decides to include the discussion of the wicked in the recitation.
Psalm 1391O lord, thou hast searched me, and known me.
2Thou knowest my downsitting and mine uprising, thou understandest my thought afar off.
3Thou compassest my path and my lying down, and art acquainted with all my ways.
4For there is not a word in my tongue, but, lo, O LORD, thou knowest it altogether.
5Thou hast beset me behind and before, and laid thine hand upon me.
6Such knowledge is too wonderful for me; it is high, I cannot attain unto it.
7Whither shall I go from thy spirit? or whither shall I flee from thy presence?
8If I ascend up into heaven, thou art there: if I make my bed in hell, behold, thou art there.
9If I take the wings of the morning, and dwell in the uttermost parts of the sea;
10Even there shall thy hand lead me, and thy right hand shall hold me.
11If I say, Surely the darkness shall cover me; even the night shall be light about me.
12Yea, the darkness hideth not from thee; but the night shineth as the day: the darkness and the light are both alike to thee.
13For thou hast possessed my reins: thou hast covered me in my mother's womb.
14I will praise thee; for I am fearfully and wonderfully made: marvellous are thy works; and that my soul knoweth right well.
15My substance was not hid from thee, when I was made in secret, and curiously wrought in the lowest parts of the earth.
16Thine eyes did see my substance, yet being unperfect; and in thy book all my members were written, which in continuance were fashioned, when as yet there was none of them.
17How precious also are thy thoughts unto me, O God! how great is the sum of them!
18If I should count them, they are more in number than the sand: when I awake, I am still with thee.
19Surely thou wilt slay the wicked, O God: depart from me therefore, ye bloody men.
20For they speak against thee wickedly, and thine enemies take thy name in vain.
21Do not I hate them, O LORD, that hate thee? and am not I grieved with those that rise up against thee?
22I hate them with perfect hatred: I count them mine enemies.
23Search me, O God, and know my heart: try me, and know my thoughts:
24And see if there be any wicked way in me, and lead me in the way everlasting.