I fail to see why a parent who wants to make this assertion can't just take a marker to their kid's shoes themselves.
Then again, I fail to see why an atheist needs to actively make the assertion this way. Strikes me as an unnecessary fight to pick. Especially since the kid is eventually going to have to make their own decision, and since kids have all sorts of incorrect and partially-correct beliefs.
I believe in laughing Buddha
'Cause there is nobody cuter
Comes in plastic, wood, or pewter
And he's good enough for me!
-- That (Real) Old Time Religion, verse 297 variant 2