What age?
What personalities?
What will they enjoy? I mean a lot of the suggestions sound not so fascinating for a kid! They have their whole lives to see things, according to what they like and when they feel like it!
My biggest cheerful childhood memories are things pretty random and generally to do with family days out nothing special. Me and brother playing in back of car with seats down driving to see relatives (80s).
Die efterling in Netherlands.
Playing space Invaders and pacman in a 'games room' with brother in some random hotel.
Ice cream on beach seaside England.
The swings me and bro went to near grans
Etc etc.
What I have grim memories of-
Really long boring walks to look at a view or rock or something.
Castles that turned out to be small hills to look at.
Being traipsed around various stately homes, gardens, massive museums etc.
Just for a different pov!