ivermectin or Eprinex (dosage is weight/size dependent remember) is the least hard/toxic for your birds so yes you can give that. Ditto rimshoes post...
Young birds having access to the ground without a mamma to show them what to eat can get in all kinds of trouble (dependent on the area...
try in the evening hours when they are normally sleeping > if she is gaping that much then that should provide you with an excellent opportunity to use a popsicle stick or such to keep her from shutting it (cradle her neck in your hand so she cant pull away while you are inserting whatever it...