I, also, would not describe Oslo as gritty. I quite enjoyed the city (2023), spent 4 nights, 3 full days and another half on arrival day. I could have stayed longer. We didn't run out of things to see. (I'm also a "slow" traveler.)
Thinking of the 7 year old, we also spent 4 nights in Bergen. I think I could have entertained mine at age 7 better in Oslo. Bergen has the Fløibanen funicular, very central and straight up the mountain to a viewing area. An easy walk through a Troll Forest is a short distance away. Day cruises through fjords leaving from central location. Museums - but none stunning that I can think a 7 year old might enjoy. There was a recent post about a new Troll Museum - I would be checking that if privileged to travel with a kiddie.
Oslo - The Munch Museum on Floor 7 has an exhibit that I think is unique in the world - Munch's "home" recreated very interactively with some highlights geared to children. Even if not interested in the other floors of Munch's art, I think that area might be of interest to many kids.
Norsk Folkemuseum is fascinating if your child likes walking into old homes, schools, churches, farms, shops. Boat ride across the harbor to Bygdøy is a plus (and easy). If you go, study the map before setting out - there are many, many options and some may be more appealing than other.
A couple interesting ship museums - Kontiki and Fram. At Fram, one can walk on the ship. Kontiki is a unique construction. Those can be fairly short visits - not large places. I found the Maritime Museum disappointing. The 3 museums are in close proximity.
The National Museum is fabulous. The website will give you an idea if it is right for your daughter, I know not all are the museum lovers I was at that age!
Vigeland Park, as noted, if the weather is good. Take a look online to see if this would appeal.
Enjoy!
