Is juniperus suitable as a substitute lawn when we have two rombustious dogs

No - it isn't as 'flat' as the name suggests, some of the branches grow up and out as the shrub matures. Sorry.

How about gravel instead of a lawn? Or restricting them to one area of your garden?

I sympathis my back garden lawn is more like a mud bath, I have reduced the area down by half so far. If your dogs like to race round the garden like mine do be prepared for gravel to be sprayed around if you decide to us it as I have.
My next project is to reduce the lawn down abit more, a patio area and a brick path along the side of the flower beds. They are just about staying off the flower beds!!