We will spend one night downtown Seattle and want recommendations for dinner. We have children and want to go somewhere casual. Any opinions?

In no particular order: For fish 'n chips or other kid-friendly seafood, also burgers etc, the old, trusty, not-the-best but not-the-worst place on the central waterfront is Ivar's Acres of Clams. A cliche, yes, but still fun for kids. Fund before- and after-dinner walking around, too. Related possibility - burgers and fries dept. - take a ferry to Bainbridge Island from downtown, eat on the way or walk 5 min. into central Winslow on B.I. (several okay places) then return, 40 min. on the boat each way. Inexpensive, not especially good food, but what a view from your dinner table.
Other possibilities include a branch of the Old Spaghetti Factory near the waterfront (yuk IMO) or, really fun if you're not used to these things, dinner in the food court at Uwajimaya Village, the newly expanded Japanese/pan-Asian grocery/variety megastore in the International District, just east of Pioneer Square. Gawk at the astonishing selection of strange (to most people) vegetables and incredible fresh, living or recently deceased fish and shellfish, then choose from a variety of Asian food stalls. Fun and educational to boot.

Only one night? I'd second above recommendation to have fish & chips on the waterfront at Ivar's. It's fish window food, but decent. More importantly, the Seagulls will catch chips on the fly when thrown into the air by the kids. Absolutely a gas!

A super place for family style italian is ....Beppo. It is located near Seattle Center, I think it's close to Capitol Hill. It was recently ranked one of the best Italian restaurants in Seattle. The portions are huge. The eggplant parm is great, if the kids like that. They are also well know for great meatballs.

Beppo is not near Capitol Hill - it is near Queen Anne. Also, FYI, it's a chain. But, it is casual and the food is good. On Capitol Hill there are a number of places: La Cantina (Mexican), Dilletantes (great desserts!), Pagliacci (pizza and salads)

I think the Ivar's suggestion is fine too, but you may want to think about Anthony's as well - on the waterfront - a bit better food IMO. Enjoy yourselves!

The Pacific Desert Company near the Seattle Center offers a great selection of deserts if you want to end the evening with some decedance.