Sheffield (but I'm biased). It's great because it grows with your children.
When they're young it has good quality theatres, great museums and art galleries, a high proportion of green spaces for a city and a great urban feel for shopping. You can shop indoors, outdoors, locally and it's all good.
When they're mid range, teenagers, it has great facilities on the door steps and lots of activities in the city. The fresh air angle is handled by the Peak District which is just on the doorstep.
When they're late teens, early twenties, it has a buzzing nightlife and is great for gigs.
It's a place where lots of students come and lots decide to settle. They move to Sheffield and never leave! I'm not born and bred myself, being a Midlander, so I'm not biased by birth.
When my dd was little I used to stand her on my living room windowsill (we have great views across the parks and cities) and tell her how lucky she is to live here. She's now 7 and I've never heard the word bored leave her lips. We're both looking forward to the summer hols here, because there's so much to do. I LOVE it!
Sorry...I waffled a bit. But I love Sheffield. South/South West especially!