There is no front of line pass merely to enter the WWoHP.
Staying onsite gives you Early Entry to WWoHP (specifically and exactly, I believe it's to all of IOA, but we used it for HP) every day. When someone staying offsite gets to WWoHP, it will already contain an hour's worth of people staying onsite.
We experienced differences in this on our trip. Arrived December 10 in the afternoon, no significant line even at that point for FJ, no line to get into the wizarding world at all. But just two days later, LONG lines by the time the area opened to the park. We got there half an hour into Early Entry, and the line for Ollivander's was 40 minutes (which we stood in). Half an hour later I can't imagine how long it was for the offsite people.
The next day we waited for half an hour at approx 10am just to get INTO the shop that's connected to Ollivander's. The two main giftshops don't sell all of the same merchandise, and we wanted something that was in that shop (should have bought it the day before when there were no extra lines to get in the doors, or when we were in there after Ollivander's anyway), so we waited.
So it really CAN make a difference.
Do the kids have interest in "touring" the FJ castle? Being there for Early Entry can make a difference with that (though I hear there's a separate line to just "tour"...seems like that might move too fast to really experience it though, to me).
And Flight of the Hippogriff is a pretty fun and VERY tame coaster...even though I never experienced a LONG line for it, using the FOTL pass for it was helpful in that it got us on even faster. And since my 6 year old (who refused to ride FJ but liked walking through the line with me) and I were riding it 14 times in a row, that became important to us.
Should the kids get interested in other things while walking to WWoHP, you might really appreciate the onsite FOTL pass. Just bypassing the wait for Cat in the Hat alone was worth it!
Universal was super-fun; we enjoyed it SO much more than we expected, and we expected to enjoy it! I wouldn't downplay how much you guys might like it...honestly, I'd stay just the one night onsite and have fun for one or two days at Universal.
