Well, on your son’s return, what is likely happening is that the Canada Workers Benefit (CWB) (aka the Working Income Tax Benefit) was added and the removed from his return. In the Income part of the return, if he had a small amount of employment income, then that would make him eligible for the CWB. But later, when you added the tuition, the CWB would have been removed, since full time students are generally not eligible for the credit.

With your daughter’s return, either she doesn’t have employment income, or too much for the CWB. Or she is still eligible to get the CWB despite being a student. Another possibility is that she never had the CWB added to her return in the first place, and her refund is coming from another source.

This CRA page has info on the CWB & eligibility for the credit: https://www.canada.ca/en/revenue-agency/services/child-family-benefits/witb-eligibility.html