Whew! That is a whole lot of miles to cover in 5 or 6 days. Especially with 3 children so young. Have you done much camping? camped with small children? Do you have a camper, travel trailor or are you pitching a tent? Have you traveled and camped with your children before this trip?
I feel overwhelmed just by reading your post.
I've traveled frequently on a similar route. As a single adult with adult friends I drove to Ashland, stayed overnight 2 nights, crossed to the coast and down to the Redwoods; then on To San Francisco. We stayed overnight in Crescent City on the way back and also Ashland That was 4 overnights going south and then back north. Travel time was 6-8 hours each day. We did stop for lunch and dinner. Although we were all independent adults who only had ourselves to take care of we were very glad to get home.
If you're planning to drive down the coast from Portland to Coos Bay, I've done that one too. It's a 5 or 6 hour trip with one brief stop for lunch. It's quicker cutting across from I-5 in Eugene or you can cut over in Roseburg.
The road up to Crater Lake is curvy and slow driving. I was motion sick. I have relatives near Bend. It's a several hour trip from Portland to Bend.
We drove to Crater Lake from Roseburg and came back the same day. We had a 4 yo and a 7 yo with us on that trip. I was exhausted by the time we got back to Roseburg.
The times I give are only estimates. It's been several years since I've taken any of those trips. Roads may be better. My experience with Central Oregon roads is that you do have to drive slower than on the freeway. You may also have construction delays. Based on my experience I think that you will mostly be traveling the whole time if you try to do all of that.
I've traveled with a 3 yo from Portland to Roseburg and although she was quite good I looked forward to the time we arrived at our destination where we stayed several days. We stayed at a motel and even tho the 3 yo had a crib with all of her own night time stuff she had difficulty getting to sleep. Lots of crying. She was happy during the day but not at night.
When I was in grade school our family camped with a 3-4 yo and a baby not yet walking. They were quite fussy as I remember.
I enjoy camping when I stay in one campground for a couple of days at the very least. To drive all day, set up camp, and leave the next day is tiring. And then to do it 3 or 4 more times feels impossible to me. Being on the go with little children would take it out of the realm of possibility for me.
If you've done this sort of long distance travel in this few of days, I'm glad to say go for it. I'm writing this in case you lack experience under these circumstances.
As to things to see and do. I think it's Bandon that has a good beach and a sea port. They have a harbor where the boats are winched out of the water up to the dock. Something like that.
There is Wildlife Safari in Winston, south of Roseburg if you go that way. You drive thru the park and see the animals in a more natural habitat than at the zoo. If you go during the morning you will be more apt to see animals close to your car. In the afternoon they find shade and nap.
I'll get out a map and see if it helps me remember more places. I've enjoyed all of my trips but I just cannot imagine doing them with 3 babies in a limited amount of time. Of course you can start out and then modify your plans based on how the babies react to traveling.
In the Redwoods there is a very large statue of Paul Bunyan and his blue ox, Babe, a redwood tree thru which cars could drive many years ago (people walk thru now) and an outdoor "chapel." I don't remember the name of the last site. And I don't know how far into the Redwoods these sites are located. As a grade school kid I liked stopping at each one.
The two older ones will be in awe of the size of the trees in the Redwoods. I still am.
I don't want to discourage you. I do hope that you have a great time. I really enjoy travel and hiking but not so much so with kids under the age of 4 or 5.