Haven't been to Blizzard Beach yet, but I think Typhoon Lagoon has a really nice kids area. Plus there is all the sand surrounding the wave pool to play in, which can be fun for toddlers. There is also the lazy river, which can be so relaxing.
I can tell you that you should skip River Country until the kids are MUCH older. The water in the lake is very deep and the current at the end of the slides is strong. The park is a lot of fun but I would only recommend it for children who are strong swimmers. They do have a toddler area, but it's not very exciting.
Have fun!