I just did the Coventry Sky Ride — four laps of the Coventry Ring Road plus some closed off roads in the centre, with my three year old son on the tagalong (not contributing to the ride as his feet don't reach the pedals yet, so I've added a bar for him to rest his feet on.)
It was really well organised, and the roads were properly closed off (except for a few bits in the centre where there were pedestrian crossings). It was particularly fun cycling round the ring road, which is one place I would usually avoid on a bike. There were plenty of children veering around, but most of the time the roads were wide enough to avoid them. Most of the time I was going faster than everyone else, despite dragging a wobbling tagalong behind me, which surprised me. I thought there'd at least be a few people trying to go as fast as possible round the ring road.
The only slightly scary bits were on a narrow cobbled section in the centre with a lot of pedestrians and slow children, where I had to unclip and be ready to stop at a moment's notice, especially as the bike is particularly wobbly at slow speeds.
I'll definitely be signing up for it again next year, and hopefully my son will be on his own bike by then.