Find and use the spare man. With an extra player there is always someone free, so the job is to recognise where the overload is, move the ball there quickly, and finish the move rather than forcing it into the crowd.
Play the 3v3 game exactly as normal, but give the team in possession a spare player so it is always four against three; the spare switches to whoever wins the ball. Coach them to find the free man rather than forcing it. As the group settles, take the spare player away and go back to the even game, which is the real test.
Four against three should score most of the time, and young teams reliably do not. The frustration is visible: they have an extra man, they can see they have an extra man, and they still lose the ball. An overload guarantees somebody is free and guarantees nothing else, because it is only worth anything if the ball reaches the free player before he stops being free.
Which is the actual lesson, and it is about speed rather than vision. The spare man exists continuously, but not the same spare man. Whoever is free changes every second or two as defenders adjust, so a team that takes three touches deciding will pass to a player who was open when they started looking.
The spare player has his own job and it is the one children get wrong. Showing for the ball, moving into a line where a pass can actually reach him, rather than standing behind a defender being technically unmarked. Unmarked and unavailable is the most common picture in an eight-year-old overload.
Worth telling the attacking side plainly: with four against three you should score most of the time, and if you are not, the problem is how fast you are playing rather than who you are playing to.
The part coaches undersell is that the defending three are getting the harder and more valuable lesson. Being outnumbered forces them to prioritise, protect the middle, delay rather than dive in, and work as a unit, which is a richer defensive problem than three against three ever presents. Rotate the overload so everybody spends time on that side.
Save it to your private drills, then change the animation, adjust positions, swap equipment, or drop it into a session block. Or start from a blank pitch and build your own.