My first thought was Amsterdam. Definitely not hilly, most attractions are easily walkable.
The center city retains a good bit of historical charm. Number of excellent museums.
I know you’ve already made your plans, but one thing to consider if you’ve got people coming in from multiple locations is see if you can pick one that has direct flights for everyone. One person missing a connection can cause havoc to logistical plans.
We just got back from Lisbon, a fine city escape for us but there’s lots of walking and hills. We loved our time there.
Oddly, Italy does not have a great deal going on at Easter. Sunday is a religious day, and the faithful go to mass, but when we were there pre-COVID, there was not a lot to see. It can be quite cold in Early April, too.
April in Paris!
Shoulder season, which is good, but likely to be a bit cold and damp – overcoat weather on some days. (Famously, the song was ‘April in Paris’ because that sounded good, actually May tends to be nicer)
Easter, which is not shoulder season for the French – you will get unseasonally long lines at anywhere open on a holiday, just as you would in the USA. Buy fast-entry tickets for Versailles (some of the generic multi-museum tickets are also fast-entry)