We did the same trip with a NB, 2 & 4 y.o. I thought the Copa Cabana Resort was great for this age especially the inside waterpark.
The weekend we went was the weekend the lake drained. We had gone with another family and my friend wanted to still see water show (there was no water) and try and help out by maintaining patronage. So, keep this in mind regarding my comment about the show.
There were two acts that I think are still in the show. Both involved stunts with acrobats that were dangerous. THe acrobast walked inside & outside a spinning wheel that rotated so it was very high off the ground. I didn't care for entertainment that relies on the danger of the sport to thrill the audience especially for young children. Walking back to the show is neat. The seating is steep and you could end up in a lot of sun. We did not see the watershow but I have heard it is very good.
THe hotel has a crosswalk over the busy street to get to the Tommy Bartlett entertainment. There is a nice playground there for the kids. It's a nice spot for a quick picnic lunch.
We stayed in the back properties of the Copa Cabana. They had porches off the backside and an inside entrance. Although we had to walk to get to the pool, I liked this layout for our family better.
This was our first time to Dells. There is a lot more to see if you drive into the town or even just walk up the road.
Hope this helps and you all have a great time.