Will the lighter color patches on child from hand foot and mouth disease go back away?

Yes: Post-inflammatory hypo- or hyper pigmentation is a very common skin reaction, following just about any irritating condition. It should eventually fade, although it may take a few months.

Most likely yes : Hypo or hyper pigmented patches can occur after inflammation. Most of these disappear over time .

Majority yes: Hand Foot and Mouth will cause greyish blisters. These resolve by peeling off within a week, usually without leaving scars.