Here's a fun, unforeseen effect!
Did you know that, in England, you're considerably more likely to be a professional athlete if you're born between September and December?. That same article continues:
This is not a one off. At a recent U17 European Championship, 75% of the footballers were born in a four month window. Similar results were detected in Canadian ice hockey and at the 2008 Beijing Olympics.
In fact, if you look across all sports at all levels, you'll find a disproportionately large representation of people born within a few months.
That seems to imply that there's a "best" time to be born if you want to be a professional athlete.
However, what's important here isn't really the month a person is born; it's the month that sport's season starts.
This is called the Relative Age Effect, and it actually affects all walks of life with a routine "season". Lets change gears for a second and look at Wikipedia's example. They state that most soccer leagues "use 1 January as their administrative cut-off date when determining an athlete's eligibility to compete in youth competitions, children born before a specified cut-off date are excluded."
So, lets consider a child born on January 2nd; we'll call him Jim. Let's also consider a child born on December 31st of the same year, and we'll call him Dan. Now, Jim and Dan both love soccer. At the age of 6, they both sign up, and since they're both born after the latest cutoff, they're put into the same year. Except, as Wikipedia puts it, "a six-year old born in January is almost 17% older than a six-year old born in December in the same year." So not only is Jim older than Dan, he's probably much larger.
So as the season starts, Jim is going to stand out considerably more; for all intents and purposes, he's a year older than Dan. Then, when the second season rolls around, Jim's still a "year" older, but now his coach picks him to be captain because he stuck out so much last year. Then the next year he's picked again, and so on. Dan never really has a chance here, beyond being an exceptional athlete.
How does this apply to my question?
Well, you've asked if children being born a certain month might be weaker, to which I'll ask: is there a routine event every year that kids participate in? Unfortunately, the harvest doesn't really line up with this idea. If anything, it suggests the opposite; kids born in the winter are born early in the harvest "selection process". When all the 6 year old make it out to the fields, your winter babies are going to outperform the rest.
But maybe there's other structure in your world. Maybe the serf mud-wrestling season starts on January 1. Maybe that's when school starts (for those peasants lucky enough to go). These would give a disadvantage to the September-December babies.
In other words, if you want winter babies to have a developmental disadvantage throughout life, have a developmentally significant routine in their lives start right after winter.