Sorry if it sounded harsh, but it seemed like you were, essentially, asking to be able to pick seats in advance even though that is not a feature of the fare you bought. Would it fair if you were able to do so because you are travelling with children, but a solo traveler who purchased the same fare cannot?

I do think there's a difference between that (pick specific seats in advance) and something like "we will put you in 3 seats together, but we won't know until check-in or at the gate which 3 specific seats." That I agree seems like a reasonable and common sense accommodation.

Incidentally if Expedia doesn't make it clear what restrictions apply to a fare, I would definitely followup with them as well that they should make it clearer in advance.