Legality of liability with a cafe? Skateboarders and injuries?

That is a huge liability that you are thinking of. Even with a waiver of liability and sign posting "skate at your own risk", you are still open for law suits. Being a business, you will have to carry a very high liability insurance, that being the insurance company knows of your plan. If you allow skating on the property, many companies will not insure you. The law suits could be extremely expensive.