Just checked the WA Marine Forecast, and the swell will be large, but dropping, from 13 feet to 8 feet, on Saturday. Wind switching from NW to SW, which is onshore. Sunday may have more favorable conditions, but I won't predict anything!
Sounds pretty gnarly for just off the beach, if you are fly fishing. There are some reports of Red-Tail Surfperch biting up and down the coast.
The forecast is often wrong, though. A good time to cast for perch in the surf off a flat beach like Grayland Beach is just prior to the low tide...look for channels between sandbars. If you take your scouts clamming for razor clams, and get done before the tide gets all the way to low, you might have an opportunity to fish for surfperch. I sometimes see people doing this, using bait.
I don't fish for surfperch there, as that stretch near Grayland State Park has a pretty flat beach, preferring other spots with a steeper beach and deeper channels closer to the beach.
Jetty fishing off the South Jetty was really crowded last weekend (someone every 20 feet for most of the length of the jetty, probably due to it being the lingcod opener here), but I found a rock to stand on and did O.K....mainly rock greenling and black rockfish...no lings. There are some striped sea perch there, too, but I always release them because they aren't that good to eat. I was bait fishing with sandshrimp, and also tossing lead-head plastics, catching some dinners. Not very many lingcod were taken off the jetty (I did see a couple of 'em), although the charters did well.
If you do choose to take some scouts surf fishing, and need to buy some terminal gear, check out the North Cove Minute Mart a couple of miles south on Hwy 105 at Warrenton Cannery Road. They have a small but decent selection of surfperch oriented tackle (hooks, sinkers, leader, etc.).
Disclaimer: I have no connection with those guys, other than stopping in to buy snacks/beverages once in a while.
One more item: If you are goig to bait fish, you can use the cut off tips of razor clam siphons (if you dig clams), or use a clam tube to get sandshrimp: their holes are found higher up on the beach than clams. Rig a sandshrimp with a bait-loop, or tie it on the hook with a couple of wraps of fine copper wire. Alot of bait fishers use small corkies, similar to steelhead bait-fishing.
The best article on fly fishing for Red-Tail Surfperch that I have read was on Dan Blanton's website. I would say that it is a "must read."
Surf perch fishing can be lots of fun but beware of the water. Sneaker waves could be a bad deal for the kids.
Because of the lay of the beach and the daily wind, fly fishing will only work if the kids are good casters and they have a rod heavier than a 6wt. Flies with some orange in it seems to be common.
If you use a spinning rod you will need something like a 9 ft steelhead or heavier rod. The shop listed above should be able to tell you about the basics. They may have frozen bait too. I would try to call them first.