Seriously, educate the 4 year old what a lot of them were made by (along with clay pits etc) and call them what everyone else will call them….bombholes……don’t make the child seem like some sort of nerd to his peers!
This.
You have an opportunity to educate your child about WW2, the Blitz etc and plant the seed early that war is horrific and maybe create a passion for history in them which they’ll take through school and likely excel at as a subject.
I know most of us want to preserve a sense of innocence in our kids for as long as possible but if they’re asking questions about it then they’re probably ready to hear the answer.
Best advice on parenting I ever read was in a psychiatric nurses office, it was the opening paragraph in a book on child development and it explained that the role of the parent is to provide a safe base from which the child can explore the world…. simple as that.
Compared to a lot of the wishy washy nonsense floating around that really struck a chord as sensible advice.