The Dublin III Regulation is a long-standing mechanism between EU Member States to determine responsibility for examining asylum claims.
The Home Office published comprehensive data on the number of transfers into the UK from other Dublin states as part of the most recent quarterly Immigration Statistics release on 22/02/2018. The relevant statistics can be found at as_22_q, asylum volume 5 at the following link:
The published statistics are broken down by each article of the Regulation and it is Article 8 which outlines the criteria for the Member State responsible for examining the asylum claims for unaccompanied minors.
It is important to note that the full data set includes children transferred as dependent family members under other articles of the Regulation, however a breakdown of these families by age is not published.