Please Help Me! Determing a MAGI for a child's unearned income

How do I find the MAGI for a child's unearned income?  I have completed her income form and ready to submit it to the IRS.    I have to have her MAGI amount to enter on my son's tax return for his insurance credit.   The popup says "Your dependent child's MAGI can be calculated by adding any amount reported on Form 1040 lines 2a to the amount on line 8b". How do you do find the MAGI when there is nothing on line 2a and no line 8b on her form 1040. Many thanks for any assistance.