You now that, childs have a lot of toys and this mean lot of battery. Standart toys batteries have limited life. But supercapacitor have 10 years life and 1.000.000 charge cycles. You can use supercapacitor in toys but this time problem is safety charge. Novadays you can find Wireless charge modules with lower price. When use wireless charge module for supercap charge and ready safety charge for child.
I use 2 serial 2.7V 10F supercap. When connect it, i have 5.4V 5F supercap set.
Wireless charge module have 5V input and 5V output.
5V is enough for my toys. You can do own supercap set for your toys with serial or parallel connection.