Let my people
Compare (Exodus 4:22); (Exodus 4:23).
Possibly Moses and Aaron shrank from delivering the message concerning the firstborn.
Related Commentaries of Exodus 5:1
Exodus 5:1
1 And afterward Moses and Aaron went in, and told Pharaoh, Thus saith the LORD God of Israel, Let my people go, that they may hold a feast unto me in the wilderness.