The doctor's letter is not relevant in this case because swine flu applies to everyone, not just you or your child, unless you have a special condition where you are allowed to travel and immune to all other Indian diseases and all other types of flu----but swine flu will kill you. (This would be hard to prove.)

If there were an official advisory against traveling to India, you would have a good case. But there's no such thing; swine flu is everywhere. You don't have any case for special treatment.

I assume you are not disputing the above, but you want someone to suggest how you could beat the rules? Some carriers have a lenient policy about doctors' letters and such (although fewer and fewer airlines are eager to give up revenue these days), but Indian carriers are particularly hard-assed. And, let's face it, many of us are constantly seeking ways to go around the rules, so you can't blame the airlines.