Central Oregon has many amazing places where you can enjoy a day of playing in the water and escape the summer heat. Two of my favorites with kids are Tumalo State Park and Sparks lake. Tumalo State Park is only 15 minutes outside of Bend there is ample parking and lots of trees to provide shade. The creek flows enough for a little floating fun but is calm enough for kids to splash around and swim in. There are also port-a-potties. It is five dollars to park for the day so don't forget to bring a little money. You can finish off your day with dinner and drinks at the Bite of Tumalo it is great!
Another amazing place is Sparks Lake. It is great for kayaking, paddle boarding, canoeing etc. You can also find a nice place to picnic on the shore. To access Spark lake you need to go up past mount bachelor and it there is an accesss road off to your left about five minutes past Mt. Bachelor. This place is truly spectacular and a must do on your to do list. Early and late are the best times to go. It can get pretty crazy busy during the day. A perfect day would be riding the Bike Park at Mt bachelor until close then paddling at sunset at Sparks Lake. Many local outfits offer day trips to Sparks Lake if you don't have your own gear.