Powering dual-voltage board from AA batteries
I think 3 AA cells is the best option, like you say. In toys powered by AA batteries, in order to avoid electrolyte leakage problems, it is good if the battery current is very low (like 15uA or less) when the batteries are totally dead. If the device turns off with an actual switch that disconnects the battery, then that is great. The current will be zero when the device is off.
But if there is some kind of &quot;sleep mode&quot; where the device turns off automatically after not being used, then you need to take care that the sleep mode current drops to a very low value when the batteries are drained.
The second big problem is avoiding reboot loops. The battery voltage will drop under load, and then rebound when the load is removed. So sometimes if you start up the device, it will begin to boot up, but then the load will cause the battery voltage to crash and it will shut down, then the voltage goes up and it reboots, etc.
The best way to avoid this is to have startup code which executes very early and prevents the device from booting when battery voltage is less than around 1.1 or 1.15 volts per cell (you can play with that threshold). During operation, you can keep running until 0.9 or 0.8V per cell before low battery shutdown. But on startup, the voltage must be at least 1.1 (or whatever). This combination will help prevent any kind of reboot loop from happening. Again, you may need to adjust the numbers a bit. But the main idea is that the min startup voltage should be higher than the min run voltage.
I know most of this is not what you asked about. But you did say you are new to this.