when a child has security sales the parent(s) can no longer report the child's income on their return. if the child's gross capital gains and other unearned income are more than $1100 (gains and losses can't be netted) then they have to file a return and include form 8615
now the goofy thing is that while a return for the child may be required based on the above criteria, there is no kiddie tax unless their unearned income is over $2200 - that's net capital gains, dividends and interest. if it is over $2200 then the parent(s) have to prepare their return because the kiddies tax is in effect and it's based in part on the parent(s) taxable income and tax.
https://www.irs.gov/pub/irs-pdf/f8615.pdf
