Of course it's a serious question, Karl, and one that deserves a serious answer. After all, the little guy deserves to keep warm, too.
Mainly, a little layering goes a long way. In cold weather, I always wear a pair of insulating briefs. I know that isn't quite the right bicycling clothes protocol, but I find it comfortable so long as the briefs in question have flat seams. Patagonia's Capilene Briefs ($17; www.patagonia.com) work great in cold weather, or warm weather when moisture management is more of an issue. Another real good cool-weather brief that adds a little extra warmth where you need it is MEC's Mid Weight Power Dry Wind Brief ($11; www.mec.ca). It's made from Polartec Power Dry, itself a good base-layer material, and adds a swatch of windproof nylon right in front. So the little guy has an extra layer of wind proofing.
You sort of imply you're wearing street trousers. That's fine, but you might want to consider carrying them in a saddlebag or courier bag and wearing cycling-specific tights. Down to around zero degrees centigrade or maybe a little lower, most “mid-weight” tights work pretty well. Pearl Izumi, for instance, makes what it calls the Therma Fleece Cycling Tight ($80; www.pearlizumi.com), which is a fine all-around cold-weather tight. Performance, an American seller of bikes and accessories, has a tight called the Triflex Tight (currently on sale for $70; www.performancebike.com) with strategically located windproof panels. Very, very warm.
Hope that helpsboth of you!