I found students of all grades have trouble with pronouncing and perceiving /p/, /b/, /f/, and /v/ so I made a couple of sessions for my 5th and 6th grade after school group.

First showed examples of how people judge your English ability heavily on your pronunciation. As Spongebob and Patrick I read the school's name and English as English/Engerlishy and got them to say who is better at English.

Stuff practising the sounds, including showing /f/ and /v/ have rabbit teeth (less protruded than rabbit but points it out) and /f/ and /v/are fricatives - can go on for a long time, /p/ and /b/ are one time plosives.

Listening Odd One Out: In pairs, students get a sheet each. One student reads each of the 3 words. Partner listens and says which number word 1,2, or 3 was different. When they are correct finish and move onto the next line.

Some tongue twisters I made up with pictures to show meaning.

Whisper lines: Ss in two lines. Each team has magnets of the 4 words: pan,ban,fan,van. I showed the Ss at one end the words in an order and they whisper it down the line until the student at the end arranges the words in the order they heard. This is very difficult so better to do maybe 2 words or 3 and build up. Fastest team to get it correct wins a point.

Don't want to be critical here, but I'm not sure it's the best idea to teach kids that their language ability will be "heavily judged" on them - essentially - having an accent.  They're never going to obtain perfect pronunciation at this stage, anyways, and all it's going to do is make them feel self-conscious about using their English.

Instead, I would focus on explaining that it's important to practice those key sounds to ensure clear communication and avoid misunderstandings.  Maybe just give some examples like "pork" and "fork" and whatnot.

With older kids, you can point out how a mispronunciation can actually cause offence.  I had a nice little chat with my students about that after a request that they name some animals resulted in one kid yelling out "Jews" when he meant "zoos".  (On that note, may I suggest adding j/z to sounds whose difference you should practice ...)