Does your DD want to keep playing? If she doesn't, then simple. Stop. But if she does, there are a couple of things you could try, which might help her to find the motivation to practice.
My DD is 5 and doing suzuki piano. She cries if she can't practice, but mainly because she wants to be part of the '100 day club'. It is something that Suzuki use as a motivator. Basically, you practice every day for 100 days (we count lessons as practice!), and then you have a celebratory party....and then you start working towards 200 days.
The other thing that works really well for my DD is points and stickers. We have a 1000 fairy stickers book, and so she gets a number of stickers after each practice, depending on how well it went. If she's really tired then we do a very short practice. If she's on reasonable form, then a bit longer (never much more than 20 minutes). We used to do the point system by literally giving points. She got points for doing things right - with five points equaling one sticker. But if she was silly, I got a point and she lost a sticker.
One other thing I would say is that the group lesson, whilst great at some levels would be complicated I can imagine. My DD started at the same time as two others, both of which are rather further forward in their lessons. Basically, my DD isn't the most focused, and she is a bit of a bouncy thing and so her concentration isn't always great (which is perfectly reasonable considering she's 5!). But I think she would be a little disheartened if she realised how much further forward the other two were.