Question to the Home Office:

To ask the Secretary of State for the Home Department, if she will make a comparative assessment of the number of children (a) detained in police custody and (b) remanded by criminal courts in each of the past three years.

The Home Office collects and publishes data on detentions in police custody, including age of the person detained. Data for the year ending March 2022 is available here:
Police powers and procedures: Other PACE powers, England and Wales, year ending 31 March 2022 - GOV.UK (www.gov.uk)

As this was the first year of data collection, not all forces were able to provide data. As such, the data are based on a subset of 26 out of 43 territorial police forces and are labelled as experimental statistics due to data quality issues, therefore should be interpreted with caution. Of the forces that provided data, there were 546,170 people detained in custody, of whom 35,114 were children (aged under 18).

Data on children remanded by criminal courts, including children remanded at a police station, are published by the Ministry for Justice. Data for 2021 are available here:
Criminal justice statistics quarterly - GOV.UK (www.gov.uk)

Magistrates courts tool: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1113402/remands-magistrates-court-tool-2021-revised-2.xlsx
Crown court tool: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1113403/remands-crown-court-tool-2021-revised-2.xlsx

Since the custody and criminal courts datasets cover different time periods, and the custody data is based on a subset of 26 forces, a comparison between datasets has not been made.