If you check out RidewithGps there are lots of great rides in the area here (northern CO in and around the Loveland, Fort Collins, Estes, Longmont, Lyons, Boulder circle) that people have mapped so you can see mileage and climb profile. The ride up trail ridge is epic and very challenging, especially with the elevation. You should be able to ride the Estes area immediately after arriving, but i’d give yourself a few days to acclimate before heading up trail ridge - many (most) visitors do get a headache and some dizziness as they approach the top of the road in their cars. Be prepared for any type of weather - at this time of year it rains daily but the storms are pretty short.
If you don’t want to attempt the epic climb of trail ridge road or Mt. Evans, you can’t go wrong with cycling through RMNP - it’s breathtaking, especially on a clear sunny day.
I think Estes to Bear Lake would be cool, very scenic and not overly difficult - but cars will be your constant companion 
https://www.bikeestes.org/2015/10/25/estes-park-to-bear-lake-in-rmnp-and-back-difficult/
That web page also shows a few other rides, including a gravel ride to Glen Haven via Dry Gulch / Crosier Mountain with an option to ride the road (Devil’s gulch) back up to Estes. I did this a long time ago (devil’s gulch road) and the switch backs are lung and leg busters.
If you fly fish, carry a backpack fly rod and small box of dry flies and tippet in your camelback pack and take the gravel road off the main road in Moraine Park and fish the head waters of the Big Thompson for a rest break. Carry a Life Straw filter in your pack and you can refill your water directly from the cold cool stream. Now that would be an epic ride right? Even if you don’t fish, i’d still carry the Life Straw and refill your water directly from a river or lake in the park during the ride.
For your kids, they just installed a new mountain coaster in Estes - it’s pretty fun - i’ll bet they’ll love it.