score:7
You can break this down into different questions:
Yes - there's nothing special about them being under 18 and the standard post-Brexit rules apply: https://www.gov.uk/guidance/visiting-the-uk-as-an-eu-eea-or-swiss-citizen
Getting them a UK passport might be useful anyway, and it'll make entering the UK a tiny bit easier, but is not at all necessary.
In the unlikely event that anyone even knows you did it, there's no law in either the UK or France that stops you leaving them appropriately looked after by someone else. Brexit has also made no difference to this situation.
It's actually quite unlikely that they would be challenged, particularly if they "look" like they are grandparents travelling with their grandchildren, but you could write a letter to confirm they have permission if you're concerned, as described in this leaflet which was linked from another answer. This also hasn't changed with Brexit as the UK was never in Schengen, though maybe the border formalities will be observed more closely now making it more likely that someone would ask questions.
Upvote:1
From https://www.gov.uk/government/publications/children-travelling-to-the-uk:
Every day thousands of children arrive at the UK border, many return from holiday with their family or with family friends. The safety and welfare of every child is of utmost importance to us and sometimes we may ask a few questions if an adult is not the childβs parent, or has a different family name.
This guidance leaflet explains why we stop and ask questions at the border and our duty under the Borders Citizenship and Immigration Act 2009 to safeguard travel and promote the welfare of children. It also describes how people can plan ahead when travelling with children and documentation we may ask for, such as adoption papers or proof of travel consent.