There are lots of great hikes around here to do. A very popular one is called Little Si, out in North Bend. Do not get it confused with the Mount Si hike - that one is WAY harder. Little Si is still a pretty difficult hike. The round-trip is a little less than 5 miles with a 1500 ft elevation gain. I would not take beginner hikers on this one. You may want to plan easier hikes to "work up" to this one. The trail is very nicely maintained but there are very steep sections where my kids had to use all-fours. When we went, my kids were 4 and 6 and they made it with lots of rest breaks. I think it took us 3 hours to get up to the top, but significantly shorter on the way down so be sure to allow enough time.
A tip on parking. There are two lots. Go past the first one, as the second one is the one right at the trailhead. If that one is full, head to the overflow lot which you already passed. It is a very popular trail so go early on a weekend. When we arrived at just after 9am, the primary lot was already full.