I think that there's a tendency to think that kids have to be playing or they won't stay interested, but a lot of times that's not true. You get them to go along with it by creating an environment in which such behavior is assumed. For example, there are a lot of regimented ballet or violin programs for children that are quite successful.
I agree, when my partner started teaching children's class it was all very playfull, with all kinds of games that were related to Aikido or learned them a basic move of Aikido. But quite soon she realized that this was not the way to go. And classes became more formal. I observed/participated in a few classes for children in Japan. These were ordinary formal Aikido lessons. The main difference being the curriculum of techniques that was available to the children. These kids were very dedicated and serious about their training. If you as an adult would not do the technique proper, they would not go down! The playful part started only after class.
Good luck !