You can see most characters in all the parks. Mary Poppins hangs out in the UK at Epcot, and Mulan can be seen in China at Epcot. Monster's Inc and Toy Story at MGM. There's a new show at MK that replace the Diamond Horseshoe that has a lot of character interaction, including Toy Story.
Here's info about Park Hoppers. Do you have Five Day Park Hopper Plus?
Five Day Park Hopper - Provides admission to Magic Kingdom, Epcot, Disney-MGM Studios and Disney's Animal Kingdom on any 5 days, any combination of parks on a day. Visits with multi-day passes do not need to be consecutive and unused days never expire.
Park Hopper Plus Pass - Provides unlimited admission to the Magic Kingdom Park, Epcot, Disney-MGM Studios, and Disney's Animal Kingdom Theme Park. Freedom to go from theme park to theme park on the same day PLUS a choice of visits to the following: Blizzard Beach, Typhoon Lagoon, River Country (open seasonally), Pleasure Island, and Disney's Wide World of Sports Complex. Visits with multi-day passes do not need to be consecutive and unused days and the plus visits never expire. You cannot park hop at the minor parks when using your Plus options. Each admission to a minor park counts as one Plus option.
Five Day Park Hopper PLUS Pass includes two PLUS visits.
I don't have small children, so can't really comment on that. However, I have heard that it's best to wait to see MK until you've visited the other parks. Some, not all, children love the magic at MK so much, the other parks can pale in comparison. You know your own kids.
Hope I've answered some questions for you.