My 7yo son has around 2k$ sleeping in "his" bank account (parent for children type account) earning notting. This is his money that he received at an age when he didn't know what it was for... I just put it the account and never bothered investing it. He doesn't know about it.
He also has his wallet and piggy bank. He gets a weekly allowance based on how he did in school/camp. He also usually get some cash at birthday/chrismas or other special occasion. He's a saver... if he gets 10$, he's put at least 2$ in the piggy bank and he never touch it so he probably has over 200-300$ in there.
I was thinking of starting him with investing. Is there an online broker where he could hold a balance of only 2k$ in his own account and not be charged expensive fee because of the low account balance ?