Is it safe to get dactinomycin while pregnant or breastfeeding?

No. This is considered a category d (fda classification) medication in pregnancy and "unsafe" for lactation purposes. So, generally speaking, I would say avoid exposure to this medication during pregnancy and breastfeeding. However, as always, risks and benefits must be weighed here. Perinatology consultation may be appropriate, especially if there are overriding benefits to mom.
Actinomycin D is. A big molecule, so it may not cross placental barrier, but it is highly unlikely that it is safe to use in breast feeding, and clearly dubious to use if a patient is known to be pregnant.