Matt Hancock announcement 16:00hrs on 21.09.20:

Grandparents and others who provide informal childcare will be exempt from coronavirus rules in local lockdown areas, Health Secretary Matt Hancock has announced.

Speaking in the House of Commons on Monday, Mr Hancock acknowledged the "concerns" about the impact of local lockdown on families' childcare arrangements

If this still leaves complications then another option is:
 If it is a single parent family then it is no change as they can have support bubble from family members (see below)

 Support Bubble: from the guidance:

What a support bubble is

A support bubble is a close support network between a household with only one adult in the home (known as a single-adult household) and one other household of any size.

This is called making a ‘support bubble’.

Once you’re in a support bubble, you can think of yourself as being in a single household with people from the other household. It means you can have close contact with that household as if they were members of your own household.

Once you make a support bubble, you should not change who is in your bubble.

So it could be that the 1 grandparent is the single adult household to the family and have set up a "support bubble".