A small-ish adult mouse is only about 17-22g. If you observe the feeding from outside the tub and are ready to intervene if you need to, I see no reason why a hatchling snake can't take it down.

Just go to the store and pick the mouse yourself. Ask when their shipping days are as they probably get mice that are closer to juvenile at those times.

Leave the snake alone for a week in the dark and then drop the LIVE mouse in at dusk or just after sunset.

GL!