Jason:
Your kid's been lucky so far, and I don't mean fortunate.
I don't often hear tell of what might be called a socially questionable behavior and immediately say, "You must be kidding." I played with a bluesman named Satan for more than a dozen years, and not everybody was in favor of that. In general, I'm a to-each-his-own kind of libertarian guy.
With all due respect, I think you're unwise to race with your son--not least because your behavior will surely inspire other slower and less coordinated runners to try the same thing, and a helpless kid, sooner or later, is sure to get hurt badly, or killed. It's just bound to happen. Not on your watch, probably, but on somebody else's.
I'm a libertarian, as I say. 'Taint nobody's business what I do, etc. I think it's a cool idea to take the kid for a ride in a jog-stroller in a park, where there's no danger of encountering cars. But competition is different--terrifically self-focused as the situation demands that we be. I assume that your kid is at least making a pretense of enjoying the ride; it's hard for me to imagine that you'd force a screaming tyke to go along with your racing plan, or that bystanders wouldn't shut you down. Still, I wonder about the power of compulsion that strong, driven runner-types--not necessarily you; those your behavior inspires--exerts over a young child who wants to please Daddy.
It sounds as though you pick and choose your races carefully, so that what you're doing is more on the order of an organized time trial with non-interfering runners ahead and behind you. That's better than deciding NOT to race with your kid in hand, but not much.
I never, ever thought I'd hear myself uttering the sentence, "That's in bad taste," but the idea of comparing race times--times pushing a kid in a stroller--well: That's in bad taste. Why not add additional categories for pushing pets (including lizards and cockatoos), potted plants, and, heck, the occasional corpse. Yes, I'm engagaging in reductio ad absurdum. I think road racing--and track racing, god knows!--is something that should be participated in by people capable of signing on the dotted line. Kids, too. Your kid DOES sign the waiver, doesn't he? Or doesn't he? I don't believe you've made that clear. If he doesn't, there's bound to be a problem sooner or later.