Long undies fit like a second skin, so bulk shouldn't be a problem. There are lots of different types about, but if you think she might grow out of them before she gets full use, you might like to think of something less expensive than silk. I got a couple of sets from Aldi here in the UK for £7.99 a set. I wear them as PJ's coz I get cold at night!
My personal experience has been that the waffle knit type in a synthetic fiber are warmer than anything else out there. Waffle knit cotton is not bad, but for the same amount of warmth it is bulkier than the synthetics. I haven't seen the Cuddl Duds that BEI Design mentioned, but they sound like they would be very good; the brushed fleece on the inside really does help make long johns warmer and more comfortable.
I've no idea what the temperatures might be in Chicago on St. Patricks, but it has a reputation for being the "windy city;" wind and humidity can certainly create a huge risk for hypothermia and general misery. I'd vote for the warmest thing you can find; it might be for only one day, but hypothermia is really dangerous.
(snip)
When we lived in Chicago I claimed there were only two seasons: Winter and the Fourth of July.
Jean M.
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