What prompts Frank to ask Nellie if he should get crosses for Dolly and Ben following Annushka's birth?

Annushka is born in late autumn of 1911 and Frank purchases a small jewelry cross for her. He does so because the babka who delivers Annushka asks Frank to. Dolly and Ben then request crosses. Frank is uncertain if he should do so and asks Nellie if he ought to obtain similar crosses for the two older children.