Comparatively speaking, when you factor in the amount of time and money spent on toys, they're actually a really poor form of investment. If you want to invest in your children's interest then you'd be better off investing your money on other things like:
* Depositing all that money into a bank account and let it build interest
* Stock market shares
* Real estate

Also, selling toys sucks! Massive waste of time! More conventional forms of investment not only tend to make a lot more earnings for you, but you can claim those earnings pretty much straight away. Want to access money from a bank account? Make a withdrawal! Stock market shares or real estate? Sell 'em!