Upvote:0
You should explain:
If you get a Schengen visa for just one day, you could travel across an internal border, apply for ayslum, and start lengthy proceeding to determine if you can stay or get deported -- and from India, there is a minimal chance that you could stay. For that reason the visa for that first day is difficult to get, getting a few days more is comparatively easy if it looks as if you can afford it.
Upvote:1
Do I need to mention it in the cover letter?
There is no harm in mentioning about the trade fair in your cover letter. You can provide the details about the fair and since you have already registered you can also show a receipt if you have gotten one.
Should I attach my marriage certificate and birth certificate of my kid?
Attaching birth and marriage certificate would put some weight to your application. These documents show your strong ties to the home country and could work as a guarantee that you will return to India after your visit.