Hi Michelle,
My family have travelled to Walt Disney World at the end of September / beginning of October many times and the weather has always been lovely and warm.
If your children don't care much for water slides, I would recommend
Disney's Typhoon Lagoon Water Park which also happens to be my 8-year old Son's favourite park at Walt Disney World.
They will be able to have a lot of fun in the Surf Pool with 6-foot waves, gently float around Castaway Creek, enjoy Gang Plank Falls family waterslide or even go snorkelling in Shark Reef.
You will find that most of the water slides are quite mild at this water park but you might want to give Crush 'n' Gusher and Humunga Kowabunga a miss as they are both fast.
I would also recommend that you bring a pair of water shoes with you each as the bottom of the wave pool has a slightly rough surface.
Wishing you a magical trip!