You can only do that if the investment income is the only income and it is all interest or dividends. If any capital gains then it can only go on the child's tax return. Note that the tax will probably be higher if entered in the parents return.
Enter under Less common income, child's income - or search topics for "childs income".
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
