There are multiple methods that could be used here, and potentially used in conjunction for greater effect.
1: Interpretive dance (AKA Semaphore): If the pixies wave their arms around then they will be of similar size to a hand, and as such can use their arms, torso or entire body to convey information in a manner similar to sign language. This would be easy for a pixie due to a better strength-weight ratio.
2: Sign language: From the human side, their hands can be used to mimic the pixie dancing and provide communication down to the pixies.
3: Speaking: Think about it. The pixies have been around as long as humans, our two languages will have developed side by side. Their language, though high and squeaky, can still convey meaning to our lower pitched ears. Similarly our lower pitched uttering can still convey information. Even if the two languages are dissimilar we should be able to pick out differences in pitch, timing and timbre from the pixies, and they in us. If the two languages have developed in concert then there is no reason that children of both races won't learn to be bilingual, speaking in their own language and listening in both human and pixie. Even hums, whistles and clicks can be used to convey an awful lot of meaning. Anyone who has brought up a child can confirm that nonsense words and seemingly meaningless syllables can actually mean something (A fact that Roald Dahl uses well in his books). Potentially you could even have this language be musical, with the various parts of pixie/human male/female combining harmoniously in conversation.
4: Drumming: Drum signals can be made on any open surface, or even on the torso of either species, and can be used audibly or just as a gentle tap. There are a variety of tapping or drumming languages available, the most well known is Morse Code, though that's not an ideal language for rapidly conveying meaning, and it's likely that more specific drumming patterns would evolve along with these two species.
Of course, a combination of these methods might arise from the natural formation of the language, so look forward to a pixie tap-dancing on your shoulder while whistling like some form of demented bluebird.