"Egy kicsi, kék béka ugrik a buszra." It jumps onto the bus from elsewhere. (Maybe it wants to get on the bus and travel somewhere)
If the frog jumping up and down while it is on the bus: "Egy kicsi, kék béka ugrál a buszon."
a buszon, since it is already on the bus
and ugrál is better than "ugrik" here because it is jumping repeatedly, not just once