Grace, Clara and æ/eɪ/ɛ allophone v. phone distinctions

I noted some time ago that Grace complained about my pronunciation of "lego" (which I pronounce as if it were spelled leggo, I suppose). Clara's now about the age Grace was then, and predictably enough, we had the familiar exchange recently:

Me: Clara, do you want to play legos (lɛgos).
Clara: Not lɛgos, daddy! Leɪgos!!!

Obviously, Katharine says Leɪgos, and that sticks with them as the correct pronunciation, even though I'm the one that plays legos more often, I think...

Yesterday, Grace and I had the following exchange (forgive my crappy phonetic transcription):

Grace: Daddy, can I have some? ( dædi, kɛn ai hæv səm)
Me: Yes you can. (yɛs, yu kæn).
Grace: Daddy! You said can (kæn) like a soda can. You meant to say kɛn.

This one's kind of cool because Grace is taking what I see as a set of allophones (kɛn/kæn) and hearing a distinction. Of course, she's right that /ɛ/ and /æ/ are distinct in lots of English words -- witness minimal pairs like fen/fan, send/sand, etc.

I've now been spending a lot more time paying attention to the word "can" as I say it. Of course, the funny thing is that saying kɛn in a stressed position is incorrect. I'm pretty sure, though, that we never say kæn in an unstressed position, saying something like kɛn or perhaps even kən or kɪn instead.

I wonder how much of this, and in how much detail, they cover in teaching foreigners English.