"peckish" is mostly used in the UK. An American would almost never say "peckish".
"Munchies" is frequently used in the US. However, it doesn't just mean "hungry". It usually means "I have just consumed marijuana, and the marijuana has made me hungry. I have the munchies." We'll also occasionally talk about having the munchies after consuming alcohol or having sex.
If we just want to say we are slightly hungry, we might say something like "I'm a bit hungry" or "I'm a little hungry".