"when" field property to apply to children?

I want to have setting in parent page “Show date field”, so that if it’s selected, children should have date field. Is it possible? Didn’t find info, but would be logical to have this feature, as same is possible in templates (I have this setting to show/hide date field for children, set in parent).

No, the when option can only react to field values on the same page.

An alternative would be to create the blueprint dynamically (you would have to hardcode the parent page in this case, though):

Recipe PR: