Should I leave it on the doorstep?

I believe the temperature to kill the virus is 140 degrees (is that F or C?) so the cake itself should be OK, it is just the handling, and your description of how you intend to proceed sounds as though that will be careful, so it is the same as delivering shop-bought goods.

I cant see a problem, except that there won't be anyone to eat it. Maybe scale it down a bit.

I would. It’s not any more dangerous (IMO) as my daughter dropping food off to us.
It’s not as though you would be making a habit of it.

How far are you driving? Any distance under 10 miles just do it. Take it a day in advance so even if the cake had been glimpsed from afar by a corona virus and it managed to land on it, 24 hours and it would be dead.

I’d think that if it was part of your daily exercise or a trip to buy food/essentials it would be ok. A special trip for that alone, then no.

Oh yes I would, it’s a lovely thing for you to do.

In fact we dropped off our little grandson’s Easter Egg earlier in the week and saw him and our daughter briefly in her front garden, at a distance of course.

We all know what we have to do and why but it’s acceptable to use our judgment and common sense as well.

Even if you did have to drive, as long as you combined it with a trip to a good shop, it shouldn't be a problem?

I'd ask his parents first. It depends on whether they're isolating - and how strictly.

Yes I think it would be fine petitpois. I made a batch of little fairy cakes for my GC last weekend. Before I started to bake, I put everything that I knew I'd need (bowls, scales, plastic storage box and bun tin etc) into the dishwasher for a full cycle and then I made sure that I wore disposable gloves throughout the baking process. Once they were ready, they were put into the sterilized container and left for them to collect next time they passed my house. They've been fine so far!

Can’t think why not, so long as you don’t have to drive it there!