during our first trip to England, we wanted nothing more than to sit in a pub and have a pub meal with the kids. Every single pub we walked into either didn't allow children at all, or had just stopped serving food 5 minutes before and therefore couldn't allow children. We never did eat in a pub that trip..
Now, we know where to look. They have to have a food licence and a no smoking section. In our local, there is a smoking section and the kids are not allowed in there, but the rest of the pub and restaurant are non smoking so kids are allowed. On school nights you'll see kids there doing homework while the parents are having their visiting time. dogs too. We have lots of dogs in our pub.
Also as Chary said, lots have gardens. Those are great and shows that they're family friendly.
More and more pubs are going family friendly - some of the pubs that turned us away two years ago, now advertise as family friendly!