Hi @BB26 , how awesome that you are taking your kids backpacking! You might want to post your question on the Portland page under Local Happenings, I suspect you will get quicker answers. Happy Adventuring!
Pamelia Lake near Mt. Jefferson is spectacular (though it requires advanced planning and permits). Highly recommended though for kids. Huge bang for the buck, as the lake is beuatiful, and it's a relatively short and flat hike into the lake and designated backcountry sites (a little over 2 miles). You can leave your gear at your tent site and spend a day hiking Grizzly Peak, which gives you spectacular views of Mt. Jefferson that you don't se from the lake. You can also get up to the PCT and other alpine lakes from there.
Closer to Portland, Burnt Lake near Mt. Hood is also nice, though a bit longer and definitely more uphill (3.3 miles ~1500ft of elevation gain). Same thing: from there you can hike Zigzag Mt. to get amazing views of Mt. Hood.
Mirror Lake in MHNF, Lookout Mountain in Badger Creek Wilderness and Boulder Lake in Badger Creek Wilderness worked really well for me an my 3 when they were 6-8 yo.