If you are on MAT leave, you cannot claim child care costs, correct?

You can only claim child care costs if you're working or going to school full time. 

Generally, childcare expenses are only valid if you’re working or attending school. However, there are exceptions. For example, if you have a temporary leave of absence from work (such as maternity leave fees you pay for your other kids’ childcare during this time may be eligible). Here's more info: https://www.canada.ca/en/revenue-agency/services/tax/technical-information/income-tax/income-tax-fol...