Staying on the timeshare side of Marriott would be ideal for a family of five.
There appears to be a way to get access to the waterpark, but it is limited and costs extra:
Vacation Club River Falls Day Pass
https://orlandoworldcentermarriott.ipoolside.com/
I have not done this.
Are you looking at prices directly from the resorts or looking to book via Redweek / Tugbbs / Vrbo / Ebay, etc. from people who rent their timeshares? This can be much more affordable.