Runny noses can simply be attributed to the cold weather. It has to do with the lining of the nasal cavity.
I can't remember it all exactly but we just learned about it in biology...
something do with the cellular and tissue struture has a hard tim working in cold weather - it's normal function is to hold nasal mucous where it belongs until you blow your nose, and explains why in cold weather we get the 'sniffles'.

FWIW i dont necessarily think this is what this kids problem is but working in childcare there are kids that look sick all the time and their parents take them to to the docs and its nothing - for reasons such as these. If you are really concerned i would be speaking to the kinder leader as well, and encourage the other parents that may be making remarks to do the same.