Defend as a connected pair. The first defender pressures and shows the attacker one way; the second covers at the right distance and angle to protect the space behind. Stay compact, do not both dive in, and win the ball to attack the opposite mini-goal.
Mark a 30x22m pitch with a mini-goal at each end, no keepers, 2v2. Teams score in the opposite mini-goal. Coach the defending pair: first defender closes the ball under control and shows it one way, second defender tucks in to cover the space and the second attacker. On winning the ball, attack the other goal at once. Play short games and rotate who starts as first defender.
Nine is about the earliest age at which cover means anything. Younger than that, both defenders chase the ball whatever they have been told, and the second defender is decoration. This format works at nine because it makes the cost of both players going to the ball immediate and obvious, which no amount of explanation ever does.
Two mini-goals are doing most of the teaching. With a single goal a pair can collapse onto the ball and still be roughly in the way. With two targets to protect, the moment both defenders commit to the same attacker there is an unguarded goal and an easy pass to it. The punishment arrives within a second, which is the timescale a nine-year-old actually learns on.
The first defender's job is the same as anywhere: close fast, slow down, show the attacker one way. The second defender's job is what the session is for. He is not marking the other attacker so much as covering the space behind his partner, positioned at an angle rather than flat alongside him, because flat means a single pass eliminates both of them.
The distance between the pair is the real coaching point and it is a genuine trade-off rather than a rule. Too tight and one ball through the middle beats both. Too far apart and the pass between them is available all afternoon. Around five yards is usually right at this age, and it changes with the size of the area.
Expect it to be untidy. Two nine-year-olds coordinating for the first time get it wrong more often than right, and the aim is only that they notice why.
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.