there are multiple issues. there's can be the kiddie tax which results in the child paying taxes on unearned income of over $2,200 (ie capital gains, dividends interest) based on the parent's tax rate. your state has a gift to minors act which must be complied with to avoid adverse income tax consequences for you. you may be the requirement to file an annual gift tax return.
Creating a joint tenancy with someone other than your spouse will result in a gift if you cannot remove funds from the account without the consent of the other joint tenant. The amount of the gift depends upon state law, but when a child is the joint tenant, the taxable gift is generally no less than one-half of the value of the property in the account. (thus 1/2 the income would be reportable by the child) the annual gift tax exclusion may not apply to this gift. However, your lifetime estate and gift tax exemption may apply. you can open a brokerage a/c for your child but usually as you as the custodian for your child under the state's gift to minors act which is not joint ownership. if the money you put in in any one year is equal to or more than the gift tax exclusion you have to file a gift tax return but you would be entitled to use the annual exclusion. remember it becomes their money and thus is no longer yours.
the other issue you would face is that the SSN on the a/c would likely be yours and thus the 1099's would be issued under your SSN. this you face issues in reporting only your share on your 1040.
