As a very general point, however high-achieving your kids are, they shouldn't restrict their choices to Russell Group universities. For some courses, they are going to have a better experience and gain more useful learning in some other institutions. Most obviously, this includes very high-reputation universities that just don't choose to align themselves with the RG (Bath, Leicester, etc.), but also ones with a very strong record of placing students for industrial years (e.g. Surrey) and others with very strong industry links that feed in to their facilities and delivery. I'm not saying that no RG universities do this but there are others that also do it, and some that do it better.
It will depend very much on what they want to study and what they want to do post graduation. For some career pathways there is a significant benefit to having graduated from a short list of high-prestige institutions but this is absolutely not universal and some of those industries/professions are starting to change how they look at applications.
At the other end of the spectrum, a regularly high-achieving student probably won't be happy studying alongside other students with a very different ethos. So a course/university that looks strong on paper but typically takes students with much lower grades might not be the right fit for this reason. But even this isn't always cut-and-dried for all courses.
Number 1 message, though, is don't take any notice of league tables.
Just for interest, I greatly enjoyed this blog post by Mary Curnock Cook (former CEO of UCAS): www.hepi.ac.uk/2022/08/20/its-time-to-talk-about-the-russell-group/. I don't think much of the RG when it comes to undergraduate education but even I've never put the boot in to quite that extent.