What total number of the (a) 12 452 072 children who received child support grants in the 2018-19 financial year, (b) 12 269 084 children in the 2017-18 financial year and (c) 12 081 375 children in the 2016-17 financial year (i) are enrolled in (aa) primary and (bb) high school and (ii) attend school regularly?