itch.io

I personally use the MIT license for most of my open software, which is pretty normal for a lot of code in the game dev industry. As far as jams go, it is frequently up to the creators or the jam creators. I have hosted game jams in the past and I am usually specific about requirements: 'Use license X if you want to participate in this jam' or 'we don't care if you release the source or not'. 

Creative Commons is great for art (which I don't really distribute as much).

Technicly you can select a license if you want when you upload any project on itch so really up do you. Personally I've not ever picked a license on any of my jam submissions mainly because they were just very small versions of what the game might be but this being an asset jam I guess that's diffrent lol

There is the possibility that we might do a part 2 were everyone uses the kits that were submitted to make some levels. You can select to opt out your kit when submitting though if you don't want your kit to be used for that. Specifically put the option in incase anyone didn't want to have their kit used for any reason :)

Up to you I guess on the license when you upload the project as submissions usually take a itch project page link I belive. Also should note it's under the Metadata tab of your project page to set the license its a bit hidden though you could always supply a license on the page and in the project files too!

TLDR;

I would most likely go with Creative Commons for the assets unless you want to keep them as your own then the all right reserved seems to be an all in one license to keep it to your self I belive. 

Personally most of my game projects and tools are just filed then under ARR just incase I plan to expand on the idea in the future and release a full game from the jam versions much like my current project.