Someone else had a very similar question (forgive me if it was you and I’m answering twice) but we’ve been going since ours was 3 and I think EP is. A fantastic addition even with younger ones.
First you’ll be surprised what the 6 year old can ride. The huge roller coasters ar out but our son could do everything else (except FJ) by 6.
Second, since here are fewer things for them to ride, my son tended to want to repeat rides right away. Flight of the Hippogriff 5x in a row. Having EP made it easy to just get right back in line and not worry about juggling wait times or skipping to another ride and going back.
Third, if there are rides you and your husband want to ride that she can’t the kid swap is nice with EPs. At UO the family all gets in line together and you enter the kid swap room right at the load area. Having my son wait in a 5 minUte ride with us was no problem, but I would have felt bad making him wait 45 minutes for a ride he couldn’t actually get on. So it allowed us to take advantage of more rides.
I know $600 is nothing to sneeze at, but for our family EPs are almost priceless.