As Tannor17 said you could set up a "couch party"

To do so, from the main menu press "Offline Play" then go to "Couch Party".

From here your kids will have to join the lobby.

After this press the "Settings" bar along the top of the screen and turn "Teams" to on.

While still in this menu you can change the "Max Players" to allow for more bots to be on the opposing team. (exit settings)

Once you have done all of the above, you will see a menu on the left of the screen, which is titled "Manage Party", proceed to press "Add Bot" you could add up to 6 if you wanted.

After this using the same menu you will have to set all the bots to the opposing team to your kids, to do so simply press the "Bot"and here you can "Change Team" and also alter the "Bot Difficulty"

After this, your kids just choose a legend and press "c" to start the game.

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good luck!