We went last Easter, DC were 5 and 8. We went Tue-Fri - 3 nights.
We did, roughly -
1/2 day Tuesday - Potter around city, lunch, Potter, dinner, buy Copenhagen cards.
Full day Wed - National Museum, Tivoli Gardens, we spent hours in Tivoli. Apart from thrbfinfsir, here is a good playground (the pancake house in the whale is not great thigh).
Full day Thursday - Boat trip (to Little Mermaid), Up the towww at Christiansborg palace, design museum, botanical gardens, geological museum (DH and I are both geologists!!)
Most of day Friday - back to national museum, Changing of Guard at the Ameliabourg palace, Lego shop
We could have easily spent more time there, but it is very expensive!
I’d recommend getting a Copenhagen card each -free entry to loads of places. free bus and train travel, boat trips.
We stayed at the Hotel Ansgar, which was well priced and central. Big family room (double plus 3 singles) ,excellent buffet breakfast.
If you have slightly fussy eaters, and a non-meat eater with you, we found a super “artisan” burger restaurant, Friends &Burgers which kept everyone happy. We had a couple of picnic meals too.