The number of children who are (a) eligible for free school meals, is published in the ‘Schools, pupils and their characteristics’ publication. The latest data shows that in England in January 2019, for all school types, 15.4% of pupils were eligible for and claiming free school meals. Data from January 2020 is scheduled for publication on 25th June 2020. The latest publication is available at: https://www.gov.uk/government/statistics/schools-pupils-and-their-characteristics-january-2019.
Note that this publication only covers England as education is devolved across the UK.
The number of children who (b) have become eligible for free school meals in each nation of the UK since March 2020 is not available.