I'm getting to this after they are probably home, so I'm not sure where things stand.
I wouldn't ask them if they took it. You found it and your daughter (at least) didn't do a very good job hiding it. So it's a good time to let them know that their rooms and their stuff (and later on, their phones and computers) are subject to parental review and investigation at any time.
I think I would do less talking at first, and more listening. I'd separate them and get each one's thoughts. It's important to know why they took it, and what they wanted to do with it. They clearly knew it was wrong - because they hid it. So ask questions that they can't answer with "yes" or "no" but which require longer explanations. As least you will know what they were thinking.
For most kids, parental disappointment is a huge deal, so don't be afraid to use that. It's also okay to say, "I haven't decided on the consequences yet. There will be some, but I don't want to respond in anger. I want to think about it." That's probably a good lesson for your husband to consider as well.
One option is to ask the kids what they think an appropriate consequence is - separately of course. If they get into the blame game of who started it, walk away. That just tells you - and you can tell them - that they are too young for the privileges they may expect or already have access to. If they are still too young to follow the rules, I would say, then they are too young for computer usage or play dates or _____ (fill in the blank with what they want). It's okay to make them do some work to earn money (for the family this time, not their own pockets) so they understand how hard it is to put away extra cash.
I always think a good consequence is letting kids know that you don't have time for X or Y because you had to do something else that they caused (search for money, think up a consequence, or other daily things like pick up their dirty laundry and find their missing shoes and so on).
I'd move the emergency stash, of course, and I'd buy a combination lockbox from the office supply store. I'd put that in something that won't interest them. But I'd also explain, in general and over time, the purpose of an emergency stash and what constitutes an emergency. It doesn't hurt for them to understand in an age-appropriate way that this is not buried treasure but rather a way that parents work to take care of their children.
If you've already talked to them, remember that it's okay to go back and say, "I've had further thoughts on this." This should probably be an ongoing discussion, same as "the sex talk" and the drug/drinking talk and a million other things.