All you need for this game are some sheets of paper, tape and some small balls, like the sort you’d use in a ball pit. Tape the paper down on a hard floor (a hallway would be perfect for this game) so that the centre of the paper pops up to form a tunnel, just like in the photo below from Basteln Ideen und Anleitungen (Craft Ideas & Instructions).
Then challenge your kids to roll the balls through the tunnels. If you have coloured paper, you can get them to match the ball colour to the correct coloured tunnel, or you can assign points-values to the tunnels based on how far away they are. If you have a child-size sweeping brush, toy golf club, or similar, you can up the difficultly level and get your kids to knock the balls through the tunnels – just like real croquet.