If you're traveling to Okayama with young children, I recommend planning a trip to Toy Kingdom, Okayama! The park isn't very large, but it's packed with toys and rides that are extremely appealing to children. It's better suited for children from kindergarten through around fourth grade. There aren't many thrill rides, and aside from the pirate ship and swing ride, there are no large rides like roller coasters, so older children may find it a little monotonous.
One thing I thought was great, though, is that children can play with all kinds of toys, including My Melody and Sylvanian Families, at no extra charge. Once you've paid the admission fee, they can play as much as they want, so overall it's a pretty good value. Individual rides cost about JPY 300 each. If you plan to go on several rides, I recommend buying the JPY 2,000 one-day unlimited ride pass. It pays for itself after just a few rides!

