Take it to management. I hope you have a good reason for not involving them in the first place because they're certainly going to ask. Making potentially illegal recordings in the workplace is not going to reflect well on you and you'll want to be apologetic about that as well.
Workplace theft is a serious issue. It's one of the few valid reasons for a summary dismissal even in locations that don't have at-will employment. You say that they might get fired, and in my opinion they certainly should, but why should that concern you? These people have made a choice to steal from their own coworkers(!). Would you really want to keep working with such a person for the next 20 years? Would you be comfortable attending a colleague's retirement party with the knowledge that he was happy to make off with your hard-earned money?
I am not qualified to judge the union aspect. While I imagine that unions won't attempt to shield proven thieves, least of all when they steal from their colleagues, I have been surprised before. If your union rep is someone you can trust who has a history of reliability then you could ask them for input without going into details. Otherwise I'd personally still go to management first, but like I said, I don't have any experience with unions.
As I mentioned, you'll want to handle this carefully when you bring this up with your manager. Not alerting them when the theft became apparent was a bad call. Setting up a camera was a worse one. You'll want to acknowledge that you handled this poorly and give acceptable reasons for not going to them directly. One excuse would be that if it turned out to be the single-parent in the group who's having trouble making ends meet, that you didn't want them to lose their job. As Steve Jessop rightly points out, you'll want to be very careful with the reasons you give. You want to have a reasonable and acceptable reason for not alerting management that doesn't further harm your reputation in their eyes. Something like "we wanted to give the thief a chance to come clean" could work, but it all depends on the particulars of the situation.
Do not go to the police without alerting management first. Your managers will not appreciate them showing up out of the blue and its management's job and responsibility to handle matters like this. You already screwed up by not involving them before now, don't make it any worse.