We are doing Venice in April with ds7 this year - he’s sturdy and doesn’t whinge so I hope the walking won’t annoy him. I wouldn’t with a 5yo. Very, very expensive and I want him to remember at least an impression of it when he grows up!
I can also recommend in highsummer: Ghent was lovely, you just need a couple of nights to see it all. Very compact and walkable. Waffles and chocolates, a river, history, a great castle. Also when we went dc liked the free scavenger hunt organised by the Tourist Office which got us rambling around the city. I don’t think it would be as much fun in the rain/cold.
Rotterdam was one of my favourite random places to visit one summer. There is plenty to do - historic tram, boat ride round the massive industrial dock and city skyline, fabulous food hall, and a great maritime museum (very hands on as your kids worksheet takes you round all the boats learning things plus a nice play area inside). We also saw the cube houses but didn’t go in to the museum. The absolute highlight was a sunny day at Plaswijkpark which is still the best value + fun park I have been to for this age group in Europe (and I’ve been to a lot). We hopped on a bus to get there. You need swimwear for the kids in the splash area.
Berlin - I’d save it for when the kids are a bit older. I’ve spent lots of time in Berlin and in my opinion 5 yo isn’t the best age to go there. It’s a superb place to visit but you get the most from it if you can absorb the history a bit and enjoy the amazing museums.
Rome - again too much for the little ones I think. I’d be so sad at missing all the big sights or dragging them round at high speed.
Brussels is dull. Paris might work …day trip to Disney?!
I second Valencia and Copenhagen, great ideas.
I also wanted to throw in a shout out for Edinburgh and York in fine weather. Great with kids!