Is eyelash enhancing serum called rapidlash safe to use while breastfeeding?

Probably safe. As a rule those product that are applied to the outside body surfaces are not absorbed in sufficient quantity to be a problem for the breast fed child. I find no specific listing of the product for this issue. Of the multiple ingredients, only licorice is a problem & then only if ingested.
Probably okay. The absorption of this into your body is likely minimal.