The most fun I have fishing all year is when my 8-year-old and I take the drift boat to Lenore in September or October.
There is a rough but well-used and reasonable launch at just about the south tip of the Island. If memory serves, it is the second turn-out from the highway after you've passed the southern tip of the lake. Trolling buggers, thin-mints, carey specials, or bead head pheasant-tail or copper-john soft hackles in #12 or #10 will keep you and the kids in enough fish to keep the kids entertained. And it's a great opportunity for the kids to hook seriously big fish that they can actually land (bring a big net; last year my boy released a 25" cut). It's a blast. Olive, browns, black, orange, and red are good, and peacock never hurts. If YOU get bored of trolling, try sight casting (particularly the soft hackles) or just working the flats and drop-offs with an intermediate line and a slowish retrieve with some pauses.
My son loves to get out on the island and hike around, something that never occured to me before. It's a blast too, and of course nothing says you can't make a cast or two from shore. The best thing about going with my son is that I'm doing things out in that beautiful and fascinating country that I had been too stupid to do in 14 years of haunting the lower Grand Coulee. It had always been fish from dark to dark, drive home on the last night hallucinating from fatigue, sun, and the throes of hysterical obsession. Now we're 4-wheeling and hiking the side coulees, discovering secret dry-falls, peering into lava sinkholes, and counting animal tracks in the white-salt beaches of strange alkali ponds the color of tomato soup. Once we climbed to the top of the eastern rim of the Grand Coulee, above the northern tip of Lenore. The look on his face was worth the climb (steep but surprizingly un-harrowing), to say nothing of the view. We've even been in the Dry Falls visitors center for crying out loud. It turned out to be entertaining and informative, in a fifteen-minute sort of way.
If the kids get antsy condsider it an opportunity. I suggest Lenore Caves. Look for the sign at the north end of the lake. It's an easy climb with paved trail, steps and rails for the first third, and then nothing harrowing about the trail itself beyond that, though you will want to keep them close, as you climb fairly high up the coulee wall. I don't remember being too awfully worried about mine (and I'm the nervous type), but then I only had one to herd. I did see another guy with his two kids, both younger than mine. The caves aren't that great but the climb and the view are fun.
A really good short (and flat) hike for kids is to walk from the NE corner of Dry Falls Lake over into the next coulee and check out the two hidden alkali lakes. One is chartreuse, the other a profoundly improbable shade of red. It's about a half-mile round trip on an easy, well-worn trail, with no cliffs to climb up or down. Park at Dry Falls like you're going to fish, go to the very NE corner of the parking area, then follow the trail; you'll see the ponds as soon as you crest the first rise. On the return walk, a loop around a peninsula on DF affords the opportunity to throw satisfyingly large rocks off a 16-foot cliff that plunges straight into water of green, indeterminate depth. The deep ker chunk and high splash seem to be real crowd pleasers. Please check for any passing float-tubers before allowing any so tempted to let fly.
Enjoy your trip. Now you've got me all excited to get out there. If you see a blue lavro rowing around with an 8-year-old smart-aleck in the gun seat offering advice to passing tubers, come over and say hi.