The obvious main risk to those children if NHS said an abrupt, 'no!' would be the unregulated online clinic and parents taking children overseas.
Such a difficult area. More widely, when a medication is not authorised in the UK, it's not unknown for a parent to take a child overseas to initiate the medication. NHS clinicians may then be in the position of needing to monitor that child while they are receiving medication that doesn't have a marketing authorisation in whichever part of the 4 nations healthcare system somebody is in.
It's not unknown for it to happen that a parent will do this, and then there is a later marketing authorisation and a positive recommendation from NICE - so in such cases, the parent acted in line with their best belief for their child. (I'm not talking about experimental or unevidenced protocols, I mean that there is trial data, possibly still at an early phase II trial or a phase III with only interim results, and treatment with it is available in some countries.)
I'm loth to see a hard stop on using NHS resources to support parents who do this for their children particularly where this is for one of the devastating childhood conditions that has a catastrophic impact on a child's survival and/or quality of life. I see this group as entirely different to the ones discussed here. I'm aware that some would argue that they're equivalent.