The Subjunctive mood can be used in object clauses introduced by the conjunction lest after verbs denoting fear (to fear, to dread, etc.).
The Analytical Subjunctive with the mood auxiliary should is used here.
Она боялась как бы они не узнали, что у нее есть ребенок.
She feared lest they should learn she had a child.
Note: If an object clause is introduced by the conjunction that the Indicative mood is used.
Она боялась, что они узнают, что у нее есть ребенок.
She feared that they would learn she had a child.