1. Lots of people think redheads are sexy.
2. The implication of "redheaded stepchild" is that "it's obviously not your kid" or possibly even a bastard.
3. I'm fat, and a Rails developer, and I don't get my knickers in a twist every time somebody makes the "fat models" metaphor about good MVC design.
No offense, but your sensitivity to your kids' redheadedness is far more likely to make them think it's a bad thing than some random colloquial phrase.
I guess I must have misnderstood the colloquialism as I read it as similar humor to this http://smh.drive.com.au/governments-dont-be-a-dickhead-campa...
You are right, the "redheaded" part of the phrase is simply meant to imply the child is obviously not a legitimate child of the parent (who is assumed to be a non-redhead). So I was way off base, my apologies.
That's what you get for posting immediately after learning your flight has been delayed two hours :-(
The point of "redheaded stepchild" is that nobody loves it cuz it's not THEIR baby. Making it the perfect metaphor for this product. :)