Rescuing my back garden

My back garden has become a muddy run from the dogs with clay-bottomed puddles that becomes baked and bare in the summer. It's also pretty uneven, with a higher bit going around the house and a lower bit bordered by sleepers. The lower bit is around 7m by 12m, and I would like to level it and get some grass on it.
I thought about levelling with a few tons of builders sand and then topsoil on top - after that I can grass it or do whatever. Fencing it off is easy enough...
Is sand then topsoil a reasonable idea, given that it would be on clay probably?

 

As you say the first thing is to keep dogs off it in the winter, it's easier if you watch this video.