Beth-
If you are going to Universal, I can't stress enough how worth it the Express Plus passes are. For an extra 15.95 pp per day you get a pass that allows you to go through the express line on each ride once. The express line is basically the same as the express passes at disney, but with the express plus passes you just hop right in line without having to come back at a specific time. We never waited more than 10 minutes for ANYTHING. If your kids really want to do the big rides, I think you could safely do both parks in one day if you wanted to fork out the extra money. You can't get the express plus online, but you can call the universal vacation planners and get the tickets in advance, or you can get them in the park first thing in the morning. They only sell a set number of them a day and they do sell out. You buy them seperate from your tickets, so it doesn't matter if you already have the tickets.
Oh- and when we went only the Shrek 4d movie actually scanned the passes, so we could have used them on the other rides more than once.