How to "block lightly"

I knit a balaclava for my babies, and the fit is currently perfect.

I know why blocking will alter the shape and fit a bit, so how can I lightly block to keep the shape it currently has?

I usually don't block hats for myself, but I know theirs will need a good wash eventually so I might as well do it now.

I used non superwash wool, (double sunday)