Grandchild is not a child, which is what the statute requires (Code Section 2) :

“(a)Definition of surviving spouse
(1)In general
For purposes of section 1, the term “surviving spouse” means a taxpayer—
(A)whose spouse died during either of his two taxable years immediately preceding the taxable year, and
(B)who maintains as his home a household which constitutes for the taxable year the principal place of abode (as a member of such household) of a dependent (i) who (within the meaning of section 152, determined without regard to subsections (b)(1), (b)(2), and (d)(1)(B) thereof) is a son, stepson, daughter, or stepdaughter of the taxpayer, and (ii) with respect to whom the taxpayer is entitled to a deduction for the taxable year under section 151.”

Do you see grandchild?  I don't.  Compare the language for head of household (and in many cases, there is no difference in tax):

“(A)maintains as his home a household which constitutes for more than one-half of such taxable year the principal place of abode, as a member of such household, of—
(i)a qualifying child of the individual (as defined in section 152(c), determined without regard to section 152(e)), . . .”

Section 152( c) then allows:

“. . .(A) a child of the taxpayer or a descendant of such a child, . . .”

If Congress had wanted to include grandchildren for surviving spouse, they could have used the same language they did for head of household.