I don't think that it is a must during a slow period and I would hate to pay to upgrade. Those things aren't cheap. I would definitely wait and see after you have been in the actual parks and upgrade if needed. We had Express since we stayed on site and it was great but with smaller kids we didn't use it to its full potential. We didn't need it at all (Mid Sept. slow time) for Spiderman, Transformers, Mummy, and most of the little kids Suess rides. Those usually took longer to walk the line than anything, usually less than 10 minutes.
We loved having it most for Flight of the Hippogryff. The wait for it was usually 30+ in the regular line and walk on in the Express line. G loved this one so we rode it a lot. We also needed it with Despicable Me unless we rode it first thing. (FYI first thing DM rides had low waits but sometimes the meet and greet with a dancing minion afterward did not have a minion. If you ride early for low waits just come in later through the gift shop and see the minion if your kids like that kind of thing.)
I think that with the smaller kids you would be fine without one. If you ride the big coasters you might need it but I think during slow season it would be a bit of a waste of money. Just my two cents