Anyway, there are plenty family friendly productions at the BT (that's what Mummy calls it) and we've seen the theatre set up with tiered seating and cushions at the front and also with lots of beanbags and cushions just over the floor.
The great thing is that if you arrive early for a family show, there is a little room with books and toys where you can wait. It's available after the show too.
The theatre itself is small, which makes the action of the show immediate and makes interaction easier. A great venue for small children (the baby just slept through!)