September is a fabulous time in Lake Tahoe. Most every year my boys (ages 6,4,1) are still swimming in the lake.
I am very familiar with where your function is located. It is in the Meyers area of South Lake Tahoe which honestly puts you quite a distance (33+ miles) from the Hyatt mentioned above.
The Marriott Timber Lodge would be my best suggestion. The staff there is wonderful and I would consider it one of South Shores more upscale properties, unless you were looking at staying in one of the casinos. They even have a kids club! The outdoor pool is smaller but very adequate.
It is in the heart of all that South Lake Tahoe offers. You would be around 10 miles away from the wedding venue on Nez Perce.
Your very close (5 to 15 minutes) from a large variety of hiking, biking and beaches.
Summer is still for the most part in full swing in early September.