Where you are hun everyone knows the dangers of 'that' river IYKWIM so i would not have an issue having a party there.
If your worried you could easily write on the invitation that you request parents to stay and assist with supervision of their child due to water safety?
Maybe you could ask the local council if whie your there you are allowed to peg off a small area - use the long tent pegs and tie some pretty ribbon to it.. tell the kids they are not allowed past the ribbon without an adult?