children and shares

Can anyone one tell me why we have to be over 18 years of age for us to own shares??
My two childreen (aged 5 and 7) have got $500 each in there bank account. They can earn $2 interest or can earn $19 to $23 a year by owning a bank share and collecting the 4 to 4.5% yeild on the share.
My husband and I thought we would teach the kids, it was better to own the bank than to use the bank and get interest they offer us. Isnt this one lifes important lessons!!
I can go as trustee but was hoping for them to enjoy watching shares either return cash or have the dividends go into more shares, in their own names and not parent's name attached