It's been a while, but I think this is incorrect and ignores the language in the qualifying widow section. By my reading the widow test specifically allows a child who would be a qualifying relative, except that they made over the limit.
The relevant section from publication 501 (2017) reads:
3. You have a child or stepchild (not a foster child) whom you can claim as a dependent or could claim as a dependent except that, for 2017: a. The child had gross income of $4,050 or more
Note that being a qualified child has no income limit, so this can only be read as referring to a qualified relative, who must be your child. You needn't actually be able to claim the child as a qualified relative (because they made too much), but if you could claim them if you ignored that part - I believe that allows you to take the widow status. You're supposed to put the child's name in on line 4, if you don't actually claim them.
