Seaside is a great place to visit with kids. Even the higher end restaurants are kid friendly. The beach is mostly sand, with a cove (rocky shore) at one end.

Activities - Enjoy anything and everything on the beach: finding sand dollars, hiking, walking, boogie boarding, surfing, clam digging, crabbing, fishing, shrimping, kayaking, or just relaxing. There are also swing sets on the beach here. Rent three wheeled low pedaled bikes called funcycles to ride on the beach, or four person bikes called surreys, or mopeds, motorized carts, or regular bikes -- all are for rent in multiple places in town. The Arcade is usually a hit, and there are also bumper cars, mini golf, and a small ride.There are lots of interesting stores on Broadway, and the Seaside Factory Outlet Center is located at 12th and Hwy 101. Attractions also include Seaside Aquarium, the oldest aquarium on the west coast, and a small museum housed in an authentic 1893 cottage.

Food - Some places locals like -
Sam's:  Close to the turn around with good prices for sandwiches, and hamburgers.
Shilo:  The view is amazing, right at turnaround, on the prom.
Dundee's Bar & Grill:  Further up Broadway, has individual pizzas, a salad bar, and other American fare. Family friendly up to 8:30 pm and there is a flat screen at every table!
West Lake:  In Gearhart has the best Chinese food.
Morningstar Cafe:  A nice place for an espresso or baked goods.
Bagels By the Sea:  A favorite quick breakfast place, all types of bagels, and usually other baked goods, bagels are baked fresh daily usually 2 times or more a day, they can make any coffee drink, and have a great interior and also a drive thru. They also make sandwiches with the bagels. Favorites are the Eggle Bagel and the Vegetarian. The warm bagels are the best.
Bigfoot's Pub & Grub:  Fun interior with a stuffed Bigfoot, "lifesize". Their clam strips are delicious and huge, and the hamburgers are also very good.
Goose Hollow Inn:  On Ave U is a nice little place with good soups and various interesting sandwiches and more, often has live music in the evenings. Fine for families though.
Pig N' Pancake:  Open all day, specializes in breakfast. Family atmosphere.    Riley's:  Full service family restaurant,  also has a full bar.                                                                                                                                                                                                                      

There is a lot to do in Seaside, but if you need more to do it is only about 15 min to Astoria, and about the same to Cannon Beach, depending on traffic of course. Your kids would probably enjoy Ft. Clatsop and Ft. Stevens St. Park (North) and Haystack Rock and other small beaches south of Cannon Beach (South).