Really pleased for you dude!
If you can find out what price the kit sells for, then I agree, work out a reasonable percentage (I'd imagine it might be about 5% maximum) or, as glacius said, give a fixed price that you sell the work for so they can use it as much as they want.
Perhaps give them an option, something along the lines of "5% commission per sale OR $xxx in total - which ever suits you best, but there is room to negotiate". Given that they are a small company my feelings would be that they will go for the 5% commission as they won't want to pay fairly high sums up front.
But whatever you decide I wish you the best of luck
