Warwick Folk Festival is my favourite festival- I love lots of others but Warwick is the only one I'd never miss.
You won't get the full effect just coming for the day though, SJ- you need to spend a day in town and in the pubs, and a day round the site, drifting between concerts, workshops, ceilis and stalls and just sitting outside a marquee and watching and listening to the folky world go by.
Lots of similarities to Towersey, so I expect you'll love it.
Definitely family friendly- I've been going with my children since they were little and they now bring their own friends and wife :)