I'm pretty relaxed about a lot of things that other people are really uptight about (ie: I let my toddler go barefoot in public/uneven surfaces, I let my kids play on stairs and if they are physically capable of climbing something, I typically let them go at it, etc, etc). However, I'm a complete fanatic about helmet use and following proper cycling rules.
I also bike, a lot. We have a 6 month old and a 2.5 year old, and we use a Chariot. I started biking with both of them when ds was not quite 5 months old. He's a hefty fella, and has always been strong, but I don't think being big means he's sturdier or anything.
I did move him from the infant sling to the child supporter before we went out biking though. I like the anti-shock properties of the sling, but felt it put him up too high in case of a roll-over. If they rolled (I can't even conceive of how this would happen), he'd be smashed in to the pavement. So, I put him down next to his sister and called it good. They don't wear helmets in there. I actually don't even understand how a child in there is supposed to wear a helmet - they end up with their heads jutting so far forward they are completely ridiculous. I count on the harness plus the metal frame to act as a "helmet". DD does wear a helmet when she's in the wee ride seat in front of me, and I always wear one.
PP, I think your event has already passed, but I hope you went and I hope it was lovely! I would just secure them well, take my time, and follow the rules of the road, as long as you don't feel they're being bounced around too much in there.