As you say, if you stick to the 'pure' eight-year-old vocabulary and voice then you are limited. I think what can work, though, is if you think in terms of the free indirect style which comes very naturally in third person: where there's an external narrator, whose voice is 'infected' by the voice and point of view of one or more of the characters, but can also slide out of that to something which we call the voice of the narrative.
So, in a first person narrative, in the background is the adult as the narrator. But that doesn't mean that the voice and point-of-view (in both the physical and emotional sense) can't slide towards child's voice as and when it suits.
This is my version of that kind of trick:
http://www.writewords.org.uk/archive/26309.asp
I don't think it
need use both tenses, though. It's a nice, fluent way of keeping the two aspects of the story separate but interrelated, and personally I use it quite a lot. But you could perfectly well just do it all in past tense, sliding the psychic distance through the range from pure, distant, storytelling narrator, into close-up pure child voice and PoV, all within the same overall narrator's voice.
I blogged about psychic distance is here, by the way, if you haven't met the term:
http://emmadarwin.typepad.com/thisitchofwriting/psychic-distance-what-it-is-and-how-to-use-it.html