An appropriately silly and childish alternative for the more stronger word, known as the Penis Promise.
Girl: "Can you promise me to not tell anyone/"
Boy: "Pee Pee Promise!"
Girl: Ok, let's kiss!
when two fine gents cross streams to solidify trust or an agreement
gent 1: "just thinking about staying in and ordering some taco bell"
gent 2: "what?! you said that you'd owe me a steak dinner tonight if the broncos won over our pee pee promise!"
gent 1: "yep yep yep sorry my bad i completely forgot about our pee pee promise"
gent 2: "phew ok. i think i'm gunna order my steak with asparagus lol"
gent 1: "good one gent 2"