Hi!
I've been to USJ twice. I am American, but have not been to the USA Universal Parks (Me and my child have been to TDR, WDW and DL though and I went to Paris DL pre-child before it was popular)
This is the USJ Crowd Calendar:
http://www15.plala.or.jp/gcap/usj/It hasn't gotten estimates for August yet. If you go back to 2019 August though I think the day was yellow or orange.
My child did not make the height line for Dinosaur last trip (June 2018) and might still not make it. (My child is petite.) We got express passes in June 2016 but not for June 2018. I don't remember a lot of single rider lines at USJ vs. TDR, but USJ has nicer waiting areas for children who don't make height lines. It's possible there are some, but I don't remember seeing them for anything we went on. When we had the express passes in 2016, my child was too short to ride Harry Potter (we had one of the 4 packs) and so I used her express pass to ride it a 2nd time. The cast members were perfectly OK with that. I also did that with the Jurassic Park water ride since she was too scared to go on it. In 2018, we didn't need passes because it was two days after a quake in Osaka and we had insane buckets of rain, so the park was completely dead. We also didn't want to do any water rides because of it and some of the coasters weren't running for most of the day. (Harry Potter was since it is indoors and I remember they got Hippogriff up and running since we went on that multiple times.) However, while the park was completely empty it took us 2 or 3 hours including waiting for a train to get to Osaka Station when leaving.
I think it really depends on what lines are like at the new Nintendo Land and how packed that is vs. the rest of the park. The same is true for TDR with its new Beauty and the Beast ride. I heard Soarin' had insane lines when it opened in July 2019.
Depending on your country, I believe JTB offers some hotel stays with 15 or 30 minute early entry for USJ. That can make an enormous difference. We do tend to stay near USJ regardless because 1) it's not that expensive and 2) I like having a short walk in the morning. I also stay on property for TDR because the 15 minute early entry is SUPER helpful. I use to think 15 minutes doesn't make a huge difference but I was wrong. Last trip it gave us a chance to go on a lot more rides at Sea which just has fewer attractions overall, so the lines can get long.
For USJ, and whether it is worth it I would say it depends on what you and your kids like in rides. USJ is definitely "wilder" than TDR. My child loved the HP rides and liked the Sailor Moon show. We also both enjoyed the VR Final Fantasy coaster, but she was disappointed she couldn't make all the height lines. I'm waiting until she can definitely make all the height lines before going back. It's also why I am waiting on Fuji Q and Nagashima. There are some kiddie rides in USJ. Overall, they have MUCH shorter lines than in TDR.
There are no kid discounts on express passes, but the park tickets for kids are discounted a lot more than in the USA. I can get a ticket for me and my child to USJ or TDR for the same price as just my ticket to DL or Universal in the USA.
Good luck!