In Peter Pan, why was Mrs. Darling nervous to have Nana outside while they were away at the party?

In Chapter One, Mrs. Darling first starts to become nervous about leaving her children alone without Nana when Wendy tells her that a boy has come to the third story window of her room at night. Mrs. Darling tries to convince herself that Wendy has been dreaming, but Wendy points out the leaves left on the window sill. The story says,

Oh, surely she must have been dreaming.
But, on the other hand, there were the leaves. Mrs. Darling examined them very carefully; they were skeleton leaves, but she was sure they did not come from any tree that grew in England.

At the end of chapter one and the start of chapter two, Mrs. Darling becomes even more nervous when she awakens from a dream about Neverland to see the nursery window thrown open and Peter Pan standing there "gnashing his pearls" at her because she is an adult. At just the right moment, Nana comes inside and chases Peter away, snagging his shadow in her teeth before he can fly away. Mrs. Darling examines the shadow and finds it strange. Later, when she shows it to Mr. Darling, he comments that it is indeed the shadow of a "scoundrel," or someone not trustworthy.
For these two reasons, Mrs. Darling fears to leave her children without Nana during the party, even with the "nightlights" or "mother's eyes" left behind to watch over them. In her experience, Nana has proven an adequate guardian over her children, and she sees the strange boy as a threat to them.