Once you have finished the tutorial and are on level 4 in Looney Tunes World of Mayhem, you are able to level up your Toons to a maximum of level 10, even if you are on a lower player level.

Once you reach level 10 as a player, then you can only level up your Toons to match your level. For example when you are level 11 you can then increase your Toons to level 11 as well.

This gives you time to level up the Toons you want while you are working on increasing your player level.