My daughter got the same thing - a big reassessment and owes $5000 + mega interest.
I have discovered the problem in Turbo Tax. The amount of childcare expenses posts automatically in to form ON479 once you enter the Receipt via the T-Slip entry screen. BUT - apparently there is another form/schedule 479-A Worksheet that needs to be completed, where the family income is calculated. Turbo Tax does not prompt to complete this worksheet, and does not give a Warning when reviewing the tax return. So nobody notices it, as it is an auto-filled form that we don't even know exists.
This causes the problem that the credit calculates only based on the one income of the spouse claiming the credit and does not account for the income of the higher income spouse.
You must complete this worksheet ON497-A, Question 3 stating 2 supporting people for all children listed as dependents, AND that one of the supporting people is 'your spouse'. Then your spouses income will be posted in, added to yours, and the grand total if over $150,000 will completely negate your child care expense credit.
Turbo Tax should either auto-fill this form, or prompt for it to be completed. This is a negligent error/flaw in the software that is costing people all this money - but legitimately owed money - although interest on it and the expected owing versus really owing being so different is VERY PAINFUL.
