Totally depends upon the location and the amount of supervision. Five is pretty young not to have a dedicated adult in charge of that one child alone.
Even assuming no wildlife (alligators, snakes, bees in dead trees on the river bank), there are always dangers of standing up in shallow water and getting a foot stuck in the rocks. Even for adults. It doesn't take much moving water to push you down. Drowning can happen. But even if not, broken ankles, scrapes and scratches, etc are also risks of bad choices in moving water where there are rocks.
Pretty much everyone should be wearing a life jacket, although most older kids and adults who can swim may choose not to do so. At your kids ages, and especially their level of swimming skills (or lack thereof), they need life jackets and close supervision at all times.
I've floated in tubes in lakes, streams, and rivers, even rafted and kayaked in white water. Water can be dangerous, despite how relaxing and fun it can be. And if you are a few feet away from someone who needs you in the water, it can be too far to be 5 feet away. Hence, I'd only feel comfortable with one-on-one supervision for the kids.
When we took our kids (2 kids) we both went (husband and I) along with a larger group of extended family. We switched off letting the youngest float with us in our tube and staying very close by the older one, never letting him get very far ahead/behind the "free" adult. Even beyond your reach, you need an adult with "free" eyes to watch for them and give them instructions and warnings, and to keep them from doing things they may not remember are dangerous (standing up on rocks in the bottom of the creek/river, when it's relatively shallow, for example).
I wouldn't not go. But I would be prepared to NOT be relaxed much, and to have a one-to-one supervision ratio for my kids. So you and husband. Or another "dedicated" adult that isn't also watching all of your friend's kiddos. You mention hers are swimmers, but not their ages. Unless they are old enough to babysit yours, then they need their mom to be eyes on them.
:)
I hope you have fun and don't forget the sunscreen, bug spray, and water bottles.