For U.K. holidays with similarly aged kids when dad wasn’t able to come, I couldn’t face long drives, so chose places easily accessible by train. London (obviously), York and Northumberland were all good options with loads to do.
Edinburgh was the most successful and was repeated many times. Very compact for walking, very safe and excellent public transport - clean buses everywhere. You have to avoid the Festival though when prices rocket and it’s mobbed. So go July or early August.
Based in Edinburgh, Glasgow is only 50 mins away on train too. In Edinburgh, we did the (excellent and, from memory no small cages/ enclosures) zoo - superb penguin parade, Dynamic Earth (Disney type science museum - moving floors, 3D movie, iceberg etc), Camera Obscura ( camera plus three floors 3D exhibits etc very fun).
Cheaper days included organising our own ‘ Harry Potter’ walking tour taking in Cockburn Street (great winding cobbled street, interesting shops, gelateria), the Elephant House cafe, George Heriots school ( from outside), Greyfriars churchyard ( lots of HP names inspired by gravestones there) (animal obsessed kids also loved standing in line to stroke Greyfriars Bobby statue’s nose!). The Museum of Scotland was free, fun and interactive (opposite Greyfriars Bobby).
On other trips, we did more outdoorsy stuff, including Arthur’s Seat, Carlton Hill ( Calton?) Hill, Blackford ( I think) Hill and lots of mooching around Princes Street Gardens.