Just by not seeing pictures, I'm in agreement with putting some evergreen shrubs behind the knockouts. It would need to be something that grows well where you live. Boxwood is popular down here. It's a can't "go wrong". Also try putting another type of evergreen shrub that is somewhat larger in the middle of the bed, such as a holly.

As for the sidewalk, I'm also a newbie. I've got my first post next to your's. But, I hear what you're saying about planting things behind the roses.

Look at what we did in our front yard. It was all grass with an aggregate area at the sidewalk to the curb. Our dd was four months old when we got our house., She's now almost 13, and our ds is almost 11. We still have a lawn area in the back yard that's fine for the kids, they climb in the tree all the time in the front along the sidewalk. We recently planted three evergreen pear trees on on side yard we've turned into a Secret Garden for the children, to create some shade canopy. We are going to plant couple of trees good the climb in on the back lawn, still figuring just what (probably apples). I'd wanted to do this for some time, DH just recently came around to the idea.

We got tired of the cookie cutter front lawn look that so many people have out here. We're probably thought of as landscaping radicals in the neighborhood (hey, our house is periwinkle blue too!), but we look how it looks now, its truly US as family! We get lots of compliments on the changes we have made over the years.

To see pics of our recent landscaping projects including the Secret Garden, and Roses we grow, ck out my Picture Trail:

careytearose

Here is a link that might be useful: look at Landscaping Projects, Favourite ROSES We Grow albums