Hi, here's some fast thoughts:
If this is a play area, I'd make it enclosed rather than open - if a fence would look out of place in your area, try to get good sized shrubs. Leave room along the driveway for something colorful, maybe put a couple big planted containers near the front entrance.
One possible location for a tree would be on the corner of street / driveway, not too close to the edge.
What are your kids' favorite activities? Mine (age 2) likes to race around the yard with his little truck, at top speed, often falling over - any hardscape projects are out of the question. The jumping pad keeps him well engaged, and he'd welcome more things to climb on. The sandbox is considered OK, but not for playing alone. Places to hide are always popular.

Thanks for the feedback. We do have some play space in the backyard, I'm just thinking more for as my kids get older and play with neighbors that the front lawn is somewhat usable and not "off limits"

A fence would be out of place in my neighborhood.

Any ideas on how to visually soften the lines of the wall and house as well as how to visually create something that draws you towards the entrance?

Seems to me you might start with pathways. When you have company, where will they park and how will they get to the house? If they will park on the front street - though that retaining wall might complicate things - then a landing pad with a curved walkway to the front door might do the job (maybe also a stairway in the retaining wall?). A pathway is also where I spent most of my childhood bouncing balls and skipping; it's handy. Even if everyone will enter via your driveway, a path from the side of the driveway to the porch might anchor your design.

As for trees, think about sight lines and view blockages; also where tree debris will fall. Generally you won't want it on house gutters or on cars. And you won't want the canopy or trunk smack in the middle of a window.

With a pathway and a tree, you may find the rest comes naturally. For enclosure, though, a low hedge would do very nicely I think.

KarinL