Rechargable batteries are okay to use in most toys. The exception is when they toy has low power requirements and will sit idle for long periods of time.
Speaking technically - Rechargable loose their charge much more quickly over time than standard disposable batteries. Older rechargeable batteries also drain much quicker than standard disposable batteries. You don't normally put a rechargable battery into a clock or smoke detector. The same principle applies to certain toys.
From a marketing point of view - Rechargable batteries can shorten the amount of play time a child will have with certain toys. Every kid hates it when the batteries go dead in a toy...
