Single-city destination for a large family?

None of those cities have a lot to offer kids but maybe Barcelona

I agree with Barcelona as there is a beach for the children.

Barcelona will be lovely in May and has kid friendly attractions like Tibidabo, Spain's oldest amusement park, and the CosmoCaixa, a large interactive science museum. Barcelona also has some great day trips around Catalonia and good connections to France and other parts of Spain.

Another idea could be Valencia, Spain's 3rd largest city, located on the Mediterranean coast south of Barcelona. Valencia will have significantly less tourists than Barcelona, but has a similar vibe, with beaches, parks, and interactive kid friendly attractions. The old river has been filled in and turned into a giant futuristic green-space. Valencia is also well connected with the rest of Spain.

Hope these give a few ideas :)

Maybe Dublin. Lots of things to do in the city and since it's a small island there are plenty of day trip options.

Rome is delightful with kids! Italians dote on them and while the kids may not love museums and churches (skip the Vatican), there are parks galore, Ascol bikes in Villa Borghese, a zoo, and daytrips to many places where half the fun is the train trip. You can pop down to Nettuno or Anzio for some waterfront time (although it is not ye beach season in Italy in May). A daytrip to Florence is reasonable enough as well.

The website Ciao Bambino has some great tips and ideas.

You should consider a reasonable city with side-trips in your situation. For instance, Vienna, Buda-Pest, Munich, Paris, all have a lot in the city itself, plus there are plenty of attractions a single day away. You could spend the first 3-4 days in the city, and then go to other locations. You don't say how long you will be in Europe, so it's a little difficult to be more specific.

This is based on personal experience, I'm sure there are plenty to add to the list.

Cities with a nearly endless list of sight options are Paris, London, Rome and Vienna.

The next category would be cities with a fairly long list of sights: Berlin, Barcelona, Munich, Amsterdam, Florence, Venice, Seville.

Cities in the south Rome, Florence, Seville might be uncomfortably hot in May.

There are also places that are generally great areas to visit via day trips. Provence and the French Riviera come to mind. In Provence (Arles was my base) day trips would be by car. In the Riviera (my base was Juan les Pins but Antibes is best) day trips would be on the local train that hugs the coast.

It's nice to be able to rent a house or apartment and let everyone decide their own schedules.

I think Barcelona would be an easy in and out, have kid's stuff and for the more adventurous, there are day trips.

If you want to pick one city for this group size, I would suggest Budapest or Berlin.

In the summer of 2011 there were four of us doing a family trip in France, three adults and the 10 year grandson, spent a nice week going from Paris to Collioure going there on the TGV and back

here's 27 more suggestions, not necessarily for couples - https://www.thisisinsider.com/best-places-to-travel-new-couples-2019-3

Thanks for all the great suggestions. We’ve decided to go to Barcelona. Your tips on that city will come in handy!

Glad we could help, let us know if you have any followup questions :)