According to Pub 929, "you may pay up to $42.50 more tax" if you elect to file the child's investment income in excess of the kiddie tax amount on the parents' return. It goes on to explain why.
Assuming the child doesn't itemize, doesn't have private activity bond tax-exempt interest, is not blind, does not have an early-withdrawal-of-savings penalty, and so on for the unusual scenarios
-- is it ever worth more, tax-wise, than $42.50 to file the child's return separately?
Suppose the taxpayer has more than one child subject to the kiddie tax rules (which changed radically, age-wise, in the last two tax years). Suppose the taxpayer is subject to AMT. Does the answer change? Was it ever significantly different in the last few decades?
Side issues, such as some states not conforming to the recent federal changes on age limits for kiddie tax, are also interesting topics to comment on.
-Mark Bole