When Pharaoh would hardly let us go. Bather, "when Pharaoh hardened himself against letting us go." At his last interview with Moses, Pharaoh had absolutely refused to let them go with their cattle (Exodus 10:24-27), and Moses had absolutely refused to go without them.
I sacrifice all that openeth the womb, being males. And being clean animals. The common sense of the reader or hearer, is expected to supply the restriction. Of my children. Rather, as in Exodus 13:13, "of my sons."