One thing you can do is get a film cannister and place some fruit fly medium from a fruit fly culture in it (larvae inc) - make a small rectangular slit in the lid and then fix horizontally high in the viv - the maggots will crawl out the slit from time to time and can be picked off by the frogs and the smell of the medium should attract the fruit flies you put in there.
Another alternative is to do the same with crushed banana - the flies will be attracted to this so craeting a feeding point high in your viv.