Seriously, people, you need water for walking a 5k? You do know that it's just a bit over 3 miles, right? Not like walking 3 miles in the Sahara Desert. YOu couldn't possible go with out it??? You're not even breaking a sweat.
And there is at least 1 water stop along the route and if it's an out-and-back (loop) than you can hit up the same water spot when you turn around and head back to the finish line. And of course, there is a ton of water available at the finish line.
Don't bring anything OP. Just have pockets for your car keys. Wear a long sleeve shirt and shorts. That should suffice if you are walking at a leisurely pace. Wear a t shirt underneath (grab the one they give you at registration) if plan on walking faster than leisurely and get a bit warm.
How old are the kids that will be with you? I'd be more worried about keeping the kids from being bored in the stroller than bringing a backpack with water (as alluded to above).