I would figure out what your kids are most interested in and be sure to do that the first or last hour of park opening. For example, my kids love running, jumping, climbing and exploring, so we did Camp Jurassic one morning at 9 am and then again one night at 8 pm. It was perfect. Practically no one else there, and we rode Pteranodon Flyers over and over with no wait. We tried it one afternoon and it was very crowded. Not nearly as nice.
My kids also loved Flight of the Hippogriff, and we rode it over and over from 8 pm until 9 pm one night.
I think it is important to visit your favorite area of the park when it is least crowded.
We just got back a couple of days ago, and, like others said, by 8 pm the posted wait at FJ was usually about 15 minutes after 8 pm. Will probably get longer as crowds increase with schools letting out.
If Gringotts is important to you, I would do it first thing. Try to arrive half an hour before posted opening, because like Disney, Universal does tend to open 10-15 minutes before posted opening time - and it will take you some time to walk to Diagon Alley. Both Diagon Alley and Hogsmeade will probably be fairly crowded most of the day. Can't avoid crowds here, but you can at least go when the crowds are not insane, like first thing in the morning and last thing in the evening.
Generally, our plan is to figure out where most people want to be, and then do the opposite of that. For example, one night IOA was only open until 9 pm, but US was open until 10 pm. At 8:00 pm, we rode HE from US to IOA with absolutely no wait because most people were heading to US to see the laser show and to stay for an additional hour. The line to ride HE from IOA to US was huge. Glad we were not doing that. Hogsmeade was not as crowded as it had been earlier in the day. FJ had a 15 minute wait, and Flight of the Hippogriff had no wait at all.