R.J.
My son in a tux (complete with vest) at age 2 is perhaps the cutest thing I have EVER seen. Especially towards the end of the day with his jacket off, sleeves rolled up to his elbows, collar open, and little red TEVAs sticking out (he did and does still have shoe issues).
If she reeeeally doesn't want one (cute cute cute), I'd say pearl grey eton suit, or the color to match the vests of the groom and groomsmen would be my 2nd choice, and black and white my 3rd.