I'm looking to make airline reservations online for myself, my wife,
and two young children.

Orbitz has a seat chart you can use to select your seats - do you
actually get the seats that you select on teh chart?

I don't see a similar seat chart function on travelocity - is there one somewhere?

Thanks.

Yes you get the seats you pick.  I've flown many flights booked on
Orbitz and it is true.

Travelocity also does support seat selection if the airline is
participating in a program to provide that choice.

and I quote, from their website Help Center:
"Can I pick my seats? 
Yes, if the airline on which you are traveling participated in
Travelocity's graphical automated seat maps feature. Today, over 30
airlines are participating, and the chances are good that your
favorite is among them. If so, on the next few pages you'll get to
choose from available and open seats on each of your flights."

That sounds good.  Now, on at least one of them - orbitz or
travelocity - when I start to go through a reservation, there's a
question about seat "preference," which makes me wonder whether the
whole thing is subject to change.

Airline reservation seat assignments are ALWAYS subject to change
whether you book online or directly with the airline.  Equipment may
change, flights may be cancelled or added, people with special needs
may need a particular seat, etc. So, if you book seats online you
_probably_ will get them but it is not 100%.