You can see how we calculate your deduction for child care expenses (line 21400) by expanding the T778 form in the Summary section. The most common reasons that line 21400 will be $0 are:

• you’re the higher-income partner. In most cases only the lower-income partner may claim these expenses, or
• your earned income is $0; the lower-income partner must claim these expenses, even if your earned income is $0

The CRA defines earned income as the total of:

• employment income (including tips and gratuities, and the non-taxable part of an allowance received as an emergency volunteer;
• net self-employment income, either alone or as an active partner (excluding losses);
• the taxable portion of scholarships, bursaries, fellowships and similar awards, and net research grants;
• any earnings supplement received under a project sponsored by the Government of Canada to encourage employment or sponsored under Part II of the Employment Insurance Act or any similar program;
• disability benefits received from the Canada Pension Plan or the Québec Pension Plan;
• amounts received under the Apprenticeship Incentive Grant program and the Apprenticeship Completion Grant program, administered by Employment and Social Development Canada