The WJ-III is nice in that the tests are totally modular -- of the main three math tests (there are several supplementary ones that aren't typically given and probably won't be in this circumstance), one is a test of basic calculation skills (untimed), one of math applications (untimed), and one a test of fluency of basic single-digit addition, subtraction, and multiplication facts (timed). It would be a perfectly reasonable choice. There would be no need to administer the reading, writing, oral language, or any of the other tests -- the WJ is designed to be used in selective fashion. There are 53 tests -- giving all of them to a kid would be silly.
The Kaufman Tests of Educational Achievement is also a fine test, and also one in which the tester can give just the sections on math.
I'm not a huge fan of the WIAT, particularly of its new iteration, but it's really not a bad test and it's more or less equivalent to the others. Whatever is convenient for the tester is probably going to be fine.
If you want something more comprehensive and less vulnerable to a kid missing signs and making other "careless" mistakes, more focused on how a kid *thinks* about math, the KeyMath-3 is superb. It's keyed to the National Council of Teachers of Mathematics standards and tests each strand (numeration, algebra, geometry, problem-solving, data, etc) separately. The only thing it's missing is a test of timed fact fluency, but (1) that may not be relevant in your situation and (2) it's easily supplemented by the fact-fluency test from the WJ. While I give the WJ in most routine situations, when I'm either trying to document a particularly unusual level of talent or where there's a concern about a possible subtle LD, the KeyMath is a *great* test for letting me see how a kid approaches mathematics from a variety of angles. (full disclosure -- my name is in the manual as a field site tester, but I receive no ongoing compensation from the company -- I'm just a happy test user.)