It depends really. There are so many kit sellers out there that there will be huge competition anyway.
When you say you are a jeweller do you mean actual jeweller or beader/bought component fitter? If the latter then wherever you sell your jewellery is a reasonable place to consider selling the kits you want to make and a lot of bead workers merge the two on their own websites and sell elsewhere under the same brand name etc. Personally, I have never thought that was a brilliant idea but it seems to work well.
If an jeweller (as in actually painstaking making everything yourself from metal or chosen materials,not buying bits and putting them together) definitely seperate the two and don't use the same identity when selling on the same platform as it would change the brand image completely.
Folksy has a charge attached to it but isn't as swamped as Etsy.
Etsy is more well known and only takes commission.
Have you checked competitors of the potential products on the sites you are considering? The saturation,prices and sales achieved? That would be a starting point in narrowing things down a bit.
Facebook may be a good help along with something like this,especially if you do party kits as well.
Personally, if I were to wishing to purchase something like this I would head straight to Amazon and not even consider somewhere like Etsy or Folksy. But that may just be me!
Also,make sure you have appropriate PPL insurance as there's nothing worse than a disgruntled parent whose child has randomly been injured by something in a kit,you can never be too careful!