Nickelodeon Universe is definitely the main attraction within the Mall of America and it's great for kids of all ages. We last visited when my boys were 4 years old and the Paw Patrol area was their favorite. At 9 years old this weekend, they loved the Orange Streak roller coaster and the Log Chute water ride which ends with a 40-foot drop and a huge splash (clothes dried quickly though).
Buying tickets can be a little overwhelming. There are two main options, unlimited wristbands or point passes. You can also buy an attraction bundle which includes an unlimited ride wristband.
I'm glad we ended up going with the point passes because the lines were very long. I would say unlimited rides makes more sense for little kids, where the attractions have shorter lines, of if you're visiting during a less busy time.
We bought our tickets at a kiosk before the rides opened and I'm glad we did because the kiosk lines quickly became very long. Lines for rides starting forming around 10:30 am during our visit on Sunday (rides open at 11:00 am). We waited about 45 minutes for each of the most popular rides.
If you're looking for a quiet space during your visit, there's one at the exit for the Orange Streak ride. My autistic son spotted the sign and it was a welcome break from the noise and chaos for all of us.