Hi flower,
It IS tough to know what makes sense for our kids (I say "our", but mine is definitely not in your league, so I can only imagine your bewilderment.)
I think its wonderful when you can use learning as an inducement.
Seems completely appropriate. (Now, the other way ("count the trains so we can go swimming"), at her age, not so much.) I think the way to approach it (and you're going to have to fine tune it depending on the circumstances) is not to point out the mistake, but do mention the right thing. For instance, when she says seventeen, you can go "Yeah, seven, eight, nine." or when she spells sub instead of bus, you can say, "cool! sub!" If she asked about it you can say something like (enunciated where appropriate, "I know it's sub and not bus because it begins with an s."
