Hi Christopher,
Whether it is swooshing down the "slopes" at
Disney's Blizzard Beach Water Park or enjoying the tropical theme at
Disney's Typhoon Lagoon Water Park, my family almost always includes a day at Disney water park on our vacations.
I suggest that you arrive at a water park as soon as it opens - even 15 minutes ahead of the scheduled opening. This way you will be ready to get in, find a place for your belongings, and head right to the attractions. Although we do not typically rent a locker at a Disney theme park, we do rent a locker at a water park. We typically bring dry clothes to change into, and it is nice to have a space to put these, as well as our backpacks, sunglasses cases, and so on.
Most of the attractions will build lines as the day goes on. Once you have your belongings settled, I suggest that you head right for the bigger attractions. The wave pool and lazy river will be very enticing - especially to your children - but hold off on those until later in the day! Your younger child will probably not meet the height requirement to experience some of the more thrilling attractions. Both of the Disney Water Parks have great kiddie areas. Since you have another adult with you, this would be a good time to split up - one of you with the older one on more thrilling attractions, and one of you with the little one in the children's area.
Your plan of a long half day at a water park followed by a relaxing evening is a good one. I often forget how much energy it takes to be at a water park - your calf muscles can get a great workout climbing stairs! I would love to help you plan more Christopher; however, I am limited in the space that I have to answer. Please do not hesitate to ask if you have any further questions!