I asked my accountant to look into this and he secured an opinion from CRA on how this works.
Essentially if you file your tax return with anything listed under Child Support Payments you will be sent a letter asking for a copy of your separation agreement. Payors cannot claim for the child they are paying for.
If you want to do it, and you have multiple children, you can arrange your separation agreement in such a way that you are paying your spouse support for some of the children, and she is paying you for some of the children, and then you can both claim one of the children you are not paying for.
Frankly I think the whole thing is a fantastic bloody mess and I don't know what the heck CRA is thinking in this. It seems to me they took the easy way out by barring payors from claiming the credit and it's just another way to weight the child support tables in favour of Mom's with sole custody. Life these days is far more complicated than this.
Here is the body of the letter my accountant received from CRA you are interested in scenario #3:
(I had to trim this post substantially because there's a size limit on posts)
Source:
CCH Tax/Federal Income Tax/Tax Window Files/Tax Window Files/2000s/2001/July/
Child Support
July 06, 2001
Window On Canadian Tax Commentary
Income Tax Act: 56(1)(b), 60(b), 56.1
Interpretation Bulletins: IT-99R5, Legal and Accounting Fees (Consolidated); IT-530, Support Payments
Information Circulars: IC 70-6R4, Advance Income Tax Rulings
Please note that the following document, although believed to be correct at
the time of issue, may not represent the current position of the Department.
In respect of scenario 1A, which involves sole custody of both children by
Mom and an agreement or order that requires Dad to pay child support for both
children, in our view, Mom is entitled to the equivalent-to-spouse amount and
Dad is not.
In scenario 1B, which involves sole custody of both children by Mom, the
agreement or order requires Dad to pay child support only for Dave and does
not provide for child support for Jane. As it is unclear how in the
circumstances only one child would be entitled to support and whether the
"wholly dependent" requirement in paragraph 118(1)(b) of the Act is met, we
are unable to comment.
In scenario 2, which involves sole custody of Dave by Mom and sole custody of
Jane by Dad (split custody), the agreement or order requires Dad to pay child
support to Mom for Dave and requires Mom to pay support to Dad for Jane, with
the support payments being offset and the difference being paid by the parent
with the liability for the higher support amount to the other parent. In our
view, both Mom and Dad may be entitled to claim the equivalent-to-spouse
amount, Mom for Dave and Dad for Jane.
In scenario 3A, which involves each of Mom and Dad having each child for more
than 40% but less than 60% of the time (shared custody), the agreement or
order requires Mom to pay a specified amount of child support for both
children and Dad to pay a higher specified amount of child support again for
both children. The payment of these two support amounts is effected by Dad,
whose liability for support is higher, having to pay Mom the difference (i.e.,
net amount) between the two support amounts. In our view, assuming that the
order or agreement imposes a liability on Mom and Dad for their respective
specified child support amount (as opposed to one liability being imposed on
Dad for the net amount), neither Mom nor Dad would be entitled to claim the
equivalent-to-spouse amount.
In scenario 3B, which involves the same custody arrangement as in scenario
3A, the agreement or order simply requires Dad to pay a specified child
support amount (which is equal to the net amount paid by Dad in scenario 3A)
for both children and does not require Mom to pay any child support. In our
view, Mom may be entitled to claim the equivalent-to-spouse amount for either
Dave or Jane while Dad cannot claim it for either child.
In scenario 3C, in which, as you stated, the "custody arrangements are
identical to scenario 3A above, except that the agreement or order provides
that, although custody is shared, Dad shall have primary custody of Jane and
Mom shall have primary custody of Dave," the order or agreement provides for
payment arrangements as in scenario 3A, except that the specified amount of
child support to be paid by Dad is specifically for Dave and that to be paid
by Mom is specifically for Jane. We presume that your reference to "primary
custody" is to the amount of time spent by the child with a particular parent,
as opposed to that parent's right to make major decisions affecting the child.
In our view, assuming that the order or agreement imposes a liability on Mom
and Dad for their respective specified child support amount (as opposed to
one liability being imposed on Dad for the net amount), Mom may be entitled to
claim the equivalent-to-spouse amount for Dave and Dad, for Jane.
We trust that the foregoing comments are of assistance.
Yours truly,
Milled Azzi, CA
for Director
Business and Partnerships Division
Income Tax Rulings Directorate