I have less issue with the unaccompanied kids with UK relatives coming than with the rest of the family following. Ideally though the relatives ought to move to France to live with the kids, instead.
I don't know how all these children have managed to travel so far alone. Some of them are reputed to be very young, In one report, a seven year old stowed away on a lorry bound for the UK and was rescued by British police when he texted to say he was suffocating - so not only did he travel all the way from Afghanistan alone - he brought a mobile phone and charger with him - and knew who to contact when he was in trouble! At seven?
The Home Office have had months, if not years, to find out if these children have relatives here in Britain. If they have then es of course they should be reunited with their families.
I will echo Dannyk13 answer to your your question regarding France.