If you're looking after your grandchildren, how can you be isolating? You can't isolate WITH them; you can only isolate FROM them.
At the moment, the UK is a 'red' country as far as the Belgian immigration authorities are concerned. If we ignore irrelevant reasons for entry, such as professional ones, here, you can only enter Belgium for "compelling family reasons" (where the list of examples within Belgian legislation doesn't seem to encompass your planned journey) or for "humanitarian reasons" (which might possibly be relevant, as it includes this: "to provide assistance or care to an elderly person, a minor, a disabled person or a vulnerable person"). You can only enter with an essential journey certificate or relevant official document to prove that one of the foregoing conditions is met.
As you'll be entering from a 'red' country, you'll have no choice but to isolate (away from your grandchildren) for 10 days.
As I see it, your only hope is that when the Belgian authorities next re-examine the categories that countries are placed into, the UK will be moved into either the 'amber' or 'green' category. The colour codes are reviewed every Sunday, with any downward changes taking effect on the following day.