Hiking:
I would head to Multnomah falls. It is a tough hike up, but if your kids are in good shape you can walk it slow and they can make it.
The Salmon River trail is nice and is an easy hike
Little ZigZag Falls is also an easy hike.
Hikes with Kids - Columbia River Gorge & Mount Hood Area Hiking - Hikes is a link to Hikes with Kids in the area.
My kids are 7 and 9 and can easily do these hikes.
There are some family rafting trips (day trips) on the Sandy River also.
Northwest Paddling » Sandy River Rafting is just one link to folks that offer those.
There is lots to do!