The distinction that the quoted statement is making is between capital gain from trading and capital gain distributions from a mutual fund. But again, that distinction only affects whether the child's income can be reported on the parents' tax return. The distinction between capital gain from trading and capital gain distributions from a mutual fund does not matter for determining whether the child is required to file. So no, trading assets would not require the child to file a tax return if the child's unearned income is $1,100 or less.
