ISO plant recommendation

Hey friends, I need some gardening advice ASAP.  So, this is the planter where Mr. Pickles found the EVILLL poison ivy.

kindly ignore the giant watering pillar of ugliness

Living as we do in a state that gets, on average, 10 inches of rain in the summer, keeping the weeds at bay is exactly like shoveling while it's still snowing.  It's a hopeless and demoralizing task.  (90% of that green stuff in the picture above shouldn't be there.)  The only way to have any kind of chance of keeping ahead at least is to crowd 'em out with intentional plants!

So... what should I plant there?

Japanese maple behind it, new sandbox to the left, dying bleeding heart to the far left

It's in the backyard, so it gets very little sun thanks to the giant tulip poplars behind us.  (which are lovely in the summer -- keep the back much cooler)

lilacs to the right

There's one very meek hydrangea in that back corner, but out of frame to the right is another planter with two more flourishing hydrangeas, so I am not attached to this spindly one at all.

Now I love creeping phlox -- I just discovered what it was this spring.  Would that be too flat and boring for the whole box?  It's 8' square.  Or maybe I could plant two things -- one that blooms in spring, one in summer?  I currently don't have anything in the back that's blooming right now -- the crepe myrtles are glorious right now but they are all in the front.  Actually, I take that back -- there is also a small hibiscus that's putting out a few tentative blooms, but they're not visible from the dining room.

(view from dining room, with "empty" box dead center)

I don't want anything too tall, since I don't really want whatever I plant to compete with the maple or lilac behind, which leaves out forsythia, which I also love.  (Forsythia is also a plain-looking hedge 11/12 of the year.)  Also, it HAS to be a perennial, since I know myself well enough to accept that I cannot keep up with an annual bed.  I have two pots lining the driveway in which I was supposed to keep fresh annuals, and they are a sad, sad sight.  (I'm not going to humiliate myself by posting a picture of THAT, nossir.)

So: flowering perennial(s), not too tall, shade-tolerant, possibly blooming mid-summer.  Oh, also not poisonous to children.  Important to put that in there too.

Recommendations?