If you ask me, anyone who agrees to sign a non-compete is either clueless or bogus. Are you going to sue them if they keep working in the cake business ? You can't tell someone they can't work after they have been around you.
You can, however have them sign a NDA ( non-disclosure agreement ) to protect your recipe / processes. But even that, you would have to prove that they are divulging your recipe or using them to open a business or selling them to competitors, etc. It's mainly a deterrent.
What you should really ask yourself is, can you afford an intern ( they aren't volunteers ). Would they help you with your workload ( if you're currently experiencing high volume ). Do you have the time and willingness to shadow them ( to minimize mistakes and to actually help them learn something, that's why they asked in the first place ).