Bitcoin mining with normal PC
Yes you can, but it isn't profitable. Mining bitcoin on a normal computer is a waste of energy, because the ASICS have much higher hash/s/kW (hash per second per kilowatt). Mining LTC or any other scrypt based coin is doable on regular desktop PCs, but only those who have a good GPU are worth it.
But then there is another part, tweaking your settings. As mining is continuously stressing your card, it becomes very hot and consumes energy. Therefor its more profitable to run at a slightly less voltage (underclocking) or at only (say something like) 80% of the maximum capacity. It requires a lot of time to get the perfect values for your system. It is an important thing to do, because the margin of profit is quite low, and you really need to push the KHs/kW to the limit to get a good financial result. You have to do that for 30 computers (mining results differ from system to system, even if they are identical, I think I read somewhere) which requires even more time.
Furthermore there are two options: there are good GPU's in the computers for people who want to game in your café or there are just normal GPU's in it, and people just browse the internet and do their thingies there. In the first case, you cannot mine while someone is playing a game because it affects the gaming experience of the customer. In the second case, the GPU is probably not good enough to mine.
So I would say no, it is not profitable to mine in your café. Certainly not CPU mining because that is useless for bitcoin (and will always be due to ASICS). GPU is possible, but because it requires a lot of time and sweat I wouldn't advise it.