typically doctor's don't do anything unless it's extensive on the face or interfering in eating. She can hold ice cubes to it periodically to decrease swelling and for pain relief. tylenol for pain relief. if you have any doubts that this is a cold sore and think it could be something else then by all means give her regular doctor a call about it all. typically children her age don't tolerate some of the otc cold sore treatments with pain relievers in them ( they can sting ) and they tend to lick their lips too often to use some of the other products out there topically.
nothing really to do except for she's old enough to teach her that if she has cold sores ( or call them a lip booboo even at her age if you want but she should start hearing the word cold sore or fever blister ) she shouldn't share drinks or kiss anyone until it's healed. she's not likely to transmit it to other body parts or even her lil sis. If you are a kissy kissy family, make a game out of "hollywood" kisses - overly exagerated air kisses with lots of noise

betsy