A good supply of filter media and media strong gravel.
If this is a serious post then eventually, however well-intended your current business plans are, when you fold under financial pressure and start selling uncycled tanks to noobs you can insist that the media and gravel are sold with the tank, cheap and if your fish aren't dying it encourages good word of mouth, you aren't gonna be advertising in even the paper anytime soon. It costs alot.
Also create your own simple print out on fish in cycling, distribute with every tank, no matter what they say, potential fish owners can be just as dishonest as FPS staff . By all means discourage fish in cycling but if you have a nice business savy head on your shoulders it'll be easy to weed out those who seem they may be planning to cycle fish-in and you could insist on a treatment/media/info based add on sale.....
Seachem prime for emergencies, simple explanation sheet with disclaimer, some bacteria ridden gravel and a nice chunk of filter media.
This could also lead you down the bespoke route which i assume with the greater levels of labour and knowledge involved means bigger profits.
Again with the whole trip out for the kids aspect, make it as comfortable for them as possible, maybe some trusting in house fish that are used to human contact and could be (under a watchful eye) touched by kids. When you have parents coming in just to show their kids the fish if you behave sensibly/sneakily (hand in hand in business i'm afraid) you should be able to create a few sales.
Good luck.... if this isn't a wind up
Quick addition (ps; you never mentioned how old you were?) Keeping pre-filled and cycled tanks for your customers would be ideal and again benefit your business in numerous ways, that is as long as you were willing to deliver and set the tanks up in property yourselves, you could hardly expect your customers to do it. It would also allow sales to noobs, as long as you provided them with the correct literature.