The following is a quote from the CNN news of October 10.
Whether you're strapping lawn chairs between the seats to keep the kids from maiming each other or secretly fantasizing about that bottle of Chardonnay you're going to reward yourself with for enduring yet another round of "The Lego Movie" during the drive, road trips with the kids can be painful.
But guess what? In their eyes, you're no rainbow-pooping unicorn either.
What does "you're no rainbow-pooping unicorn either" mean?
Best Answer
Here the word 'their' refers to the children and 'you' refers to the adults.
We do not think our children are perfect and this sentence reminds us that, in the eyes of our children, we are also far from perfect.
Children can be thought of a pure and innocent little creatures until one becomes a parent and realises how devilish they can be (hence the reference to maiming each other).
In mythology the unicorn is often associated with innocence and purity.
This humorously suggests that unicorns are so pure and wonderful that, even when they defecate, they poop rainbows instead of faeces. Rainbows differ from faeces in that they are beautifully coloured and have no smell or unpleasant substance. Marvelling at rainbows is considered to be one of the innocent delights of childhood.
Thus the sentence says that children do not see adults as perfect, unblemished beings. The addition of 'either' indicates that the feeling is mutual.
EDIT 17 Oct 2015
As someone has pointed out, "You're no X", "He's no Y", etc. is a common idiom, e.g.
"A lot of teams out there need pitching, and by all means I'm no slouch. I've got a lot of good years left in me." Yankees '98: Best Ever By N. Y.) Daily News (New York)