We have had annual passes at UO since my child was a toddler. In “normal times” there is a wonderful building open with lots of interactive dinosaur activities near the Jurassic Park ride. They have an “ultrasound” where kids can sort large dinosaur eggs, interactive fossil activities, huge dinosaurs with kid size viewers and microphones that you can use to see out the dinos eyes and make them “talk” and an interactive exhibit where baby dinos hatch in a lab and one child is selected to name it. (You get a certificate and can request updates about your Dino with the Number on future visits and they will show you videos of your baby Dino aging). Sadly this was all closed because of Covid-19 and I’m not sure when you are going but this was my daughters favorite spot in the park. There is also a big outdoor play area near pterodon flyers (kid ride) with tunnels and fossils where they can run around and explore (kind of like the Dino area at AK). In terms of rides and shows there are a lot of height restrictions, but minions ride, shrek show are good at those ages. The simpsons area has carnival games and the TM will tell you which ones “always win” if you have small kids. There’s a ride like dumbo but with the aliens from simpsons and the dialogue is hilarious. There is another play area back by woodys roller coaster (also for littles) including a water feature near ET. Last time we saw a very small child like a year old on ET and were surprised, so apparently really small kids can go on it. But the problem is that the play areas, Dino building and interactive water features were all closed when we were there in June/sept 2020 and still closed in January 2021, and I’m not sure when they will be open. There is another dumbo type ride in Seuss landing and a cat in the hat ride that I personally find creepy haha. There is also a very very slow trolly ride there. Face painting was still available and they had modified it for masks, so it’s just top half of the face. In the Harry Potter areas your 6 year old can do the interactive wand and not sure ages of your other kids but my daughter liked doing the spells at 5. The roaming wizard professors will notice small children and “help” them do spells (we have had this happen in less crowded times). When we were there the Olivander show was made into a single family event due to covid so you may want to check if that’s still a thing since it means you can pick which of your kids is “selected” by the wand. Just a note that some kids get scared when the dragon breathes fire, it’s loud. There’s a small roller coaster (hippogriff) in the IOA side of Harry Potter that has a lesser height restriction.
EDIT:because I remembered more stuff