This is what the Instructions say:

Enter “0.50” in columns (e) and (g) of the appropriate line in Part IV to allocate the enrollment premium and APTC. Leave column (f) blank because you do not allocate the applicable SLCSP premium.

Instead, enter the SLCSP premium that applies to your coverage family on lines 12 through 23.

On the 1095-A worksheet, look at the section under line 33.  Enter (1) the SSN of the ex, (2) 01 (3) 12 (4) 50 (5) leave blank and (6) 50.

The go back up to lines 21-33 of the 1095-A worksheet.  Columns A and C should show what the actual 1095-A says.  But in column B, don't enter what the 1095-A says.  You (or the client) needs to look up the proper amount that applies to her and the kid (so not including the ex).  If the use the Federal Marketplace, that can be found here: https://www.healthcare.gov/tax-tool/#/