Determing a Child's MAGI

How do I find the MAGI for a child?  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 e=when there is nothing on line 2a and no line 8b on form 1040.  Many thanks for any assistance.