My daughter play with the cat for about 15 minutes,the batteries get hot,and the plastic case close to the negative pole get melted and pop out. I fix it with hot glue and cut the springs into half length. I also notice that the number 12servo connected to pin 7 is hot even just turn on the power for ten seconds,but it is normal in function.
I know the reason for the hot servo now. When I turn on the power and hold the cat with its head up, it will keep a gesture to keep balance, and its front left shank keep rising up against its should, so the servo gets hot in a short time.
The battery holder on Nybble is not designed for carrying large current for too long. It's better to limit continuous walking time within 30 seconds. For now, you could solder thicker wires on the poles to reduce contact resistance. I hope to find a factory to customized the wires once I return China.
I originally designed Nybble to be a programming platform rather than a remote controlled toy. You can try to teach your daughter change the postures in Instinct.h. It will be more enjoyable and educational to teach Nybble new movements.
The servos should not turn too hot unless working for a while, or gets stuck by obstacles. I'm not sure about your case because number 12 servo is not connected to pin 7. Please refer to this wiring cartoon to check your wiring orders.