This is EXACTLY what I would look for over 7 years! ... Light, cotton (happening at any time). Plain, simple but effective (sober and correct, goes everywhere). With zipper (not to be c ****) and a small folding collar if necessary (perfect).
One small problem is a US size, so even the S size is slightly larger (but quite portable, a small passage to the machine "shrink clothes" -Washing and will be good).