I actually just answered a similar question, so you’re not alone in your love of lakes! For a family that size, I would suggest renting cabins at either Lake of the Woods or Fish Lake, both located on Highway 140 between Medford and Klamath Falls. Both have rustic cabins situated around the lake, and both have nice swimming, fishing, and kayaking onsite. You can also find hiking trails in the nearby Sky Lakes Wilderness. Book early, as these resorts do fill up fast for summer!
If you want something a bit more comfy, I’d recommend The Running Y Ranch in Klamath, though the lake it’s on (Klamath Lake) does not offer any swimming. Have fun!