The older they are, the harder the transition will be. You must be consistent. You slip up one time and it's worse than starting from scratch because they'll test even more. After all, you caved in one time so if they keep at it you'll cave again.
It's simple in theory, but exhausting for you and your husband. And you both must be on the same page. So the simple answer is this - just walk them back to their rooms everytime they come out before a certain time in the morning. Put a clock in their room. Even a 3 year old can "tell time" if you draw a picture of "7 o'clock" and tape it to the wall next to the clock. (my kids learned within weeks of their 3rd birthday). They come to your room, you say "it's not time to get up yet" and walk them to their room. When they leave (which will probably be on your heels), take their hand, walk back to their room and DO NOT TALK. Do not engage them. Remain calm. And yes, you'll probably have to do it 50+ times the first night. Start on a Friday night so you and your husband can take turns napping during the day. The first few nights will be rough but you may be surprised how easy the 2nd or 3rd night will be. Consistency is key.
It also helps to sit down with the kids before bedtime the first night and explain "this is the new house rule - everyone in their own bed".