Don't let's" said Kathleen

I would like to know the meaning of Don't let's.
The context from The Enchanted Castle by Edith Nesbit:
Gerald the eldest tells his brother and sister to march and they reach a dark place with steps down:
"There are steps down", said Jimmy.
It is an ice-house", said Gerald.
"Don't let's", said Kathleen.

I have to explain that she is not very brave (as girls used to be depicted in books on those days.

Maybe she means: let's not go any further?

My Many thanks
miri

The negative of _let's_ is formed in three ways: _let's not_, which is widely used; _don't let's_, which is chiefly found in British English; and _let's don't_, which is an Americanism.
From _Webster's Dictionary of English Usages_

If you are suggesting that you and someone else should not do something, you say *let's not*.
"Let's not talk about it."
"Let's not waste time."
Some British speakers say *don't let's*.
"Don't let's tell anyone."
From _Collins Cobuild English Usage_