Fears that formaldehyde from vaccines may cause cancer are similar to fears of mercury and aluminum, in that they coalesce around miniscule amounts of the substance in question, amounts considerably smaller than amounts from other common sources of exposure to the same substance.
As for mercury, a child will almost certainly get more mercury exposure from her immediate environment than from vaccination. This is true, too, of the aluminum that is often used as an adjuvant in vaccines to intensify the immune response.