To say no to this party invitation

[concernedforthefuture]

DS(9) has just been invited to a friend's birthday party this weekend (it will be a garden party).
The host family have 2 children (inc birthday child) plus Mum and Dad. I'm not sure how many children are invited but DS isn't a very close friend of the birthday child so I'm assuming more than 2, which means that it's likely there will be more than 6 people present.
So therefore going to the party would be against the rules, right (we're in England)? Birthday mum has gone to great lengths to explain how she will supply hand gel, encourage the kids to keep 2m apart etc. and they have an outside loo so no-one needs to go into the house. Although their garden isn't huge, I'm not particularly concerned about the risk of catching CV but it doesn't sit right with me - this shouldn't be happening.
AIBU to say no? Or even check how many would be there perhaps without looking rude? If more than 6, how can I politely decline without upsetting the mum or birthday child by making it sound that I'm being critical of their choice to host a party when it's currently not allowed?

[Gulabjamoon]

Meh, I'd let him go. So many kids back at school today, no way will they keep 2m apart at all times.

I think you're overthinking it as 'the right thing to do.' Just go with your gut.

[Gulabjamoon]

Got an instant response: "suit yourself!"

What were you expecting? Seems a reasonable response.