It sounds to me like they got into bad habits in the first couple of weeks and sort of decided that they could do what they like at dancing. I've seen it before with student teachers at school who don't want to be tough when faced with challenging behaviour because they want the children to like them and have fun. It can then be a little overwhelming for children of this age when the student does start to clamp down - they thought they had things sussed and you're changing the rules, hence the crying.
What I would do now is, above all, make a big fuss of any desirable behaviour there and then. Lots of verbal praise followed by a sticker, even if it's interrupting what you're doing. "Jessica came straight to the front when I asked her! Well done Jessica, that makes me so happy! I think you deserve a sticker for doing what I asked straight away!" Forget about stickers at the end of the lesson. They're with you for such a short time they need to see straight away what type of behaviour you want from them. Sadly, these days stickers are not really that exciting to children and I imagine at the end of the lesson when they're not going to see you again for a week, they can pretty much shrug off not getting a sticker at the end. But if they do something well and can go out and say "Look mum, I got a sticker from Miss Moon because I was sitting nicely!" then it might start to sink in for everyone. That way it is also about praise for desirable behaviour and leaves out any sense of punishment for negative behaviour if someone doesn't get one.
In this vein, I would also try praising others if one child is not cooperating. If Molly tries to wander off or is wriggling about, literally name check everyone else who is doing as asked. "Jessica's standing nicely, so is Amy, well done girls. I see Chloe standing very nicely too..." Sometimes this can be enough to pull the wanderer back into line.
If one child is resolute and will not cooperate despite praising others, I would give one chance "Molly, you need to come and join in sensibly or you will have to sit out." If she cooperates, a low key "Good choice Molly," is enough (or they will all be tearing off so you make a fuss of them!). If she continues, be swift and firm, take her hand if necessary and take her to sit at the side. If she protests, calmly say "You need to sit here until you can join in sensibly." If she counters, "Show me you can be sensible by sitting here nicely." And return to the other, ignore any further protests or crying. Be as sweet, happy and enthusiastic with the others as you can. Make Molly think you're having fun so she wants to join in. When she does sit nicely for a minute or so invite her to return with no praise for sitting nicely. "Are you ready to join in sensibly now?" Be consistent and follow through. It would be nice if positive behaviour management worked every time but some children need to see that there can be consequences if they misbehave.
I hope this isn't patronising. Perhaps you've done all this and you've struck it unlucky with the combination of children in you're class. I have sometimes found myself get into a sort of negative groove with a class and need to pull myself back by reminding myself to praise, praise, praise and act quickly on any outright defiance or rudeness.