Step on a crack, break your mother's back. It's old, been tried and is tired. It's the 21st century -- a sidewalk crack isn't going to hurt anyone. You need something new--something different.
Look around your garage. If your family moves often, that will help. If you have a dad and grandad that might have built a custom crate for some of the items you've moved a few times--that would be ideal!
Wait until you and dad have opened the crate and removed all the toys you need in your new garage. Go ahead and play with your sister--act like having the scooter and the hula hoops out for the first time in months is enough diversion and your parents will let you two play in the garage unsupervised. After you've played for a few minutes, it is time to put the plan in action. If your sister is old enough to figure out how to pry the nails up from the plywood with the back of the hammer, this will save you much time.
The key is to do this quietly so that mom will wonder what you're up to and will come out to the garage--you need her to come to you for this!
When she wonders how you got the lid open again and extolls the dangers of playing near an open box with sharp nails protruding from the lid, simply explain that you desperately need a tire pump that is clearly not in the box.
When she looks once more to appease you, try to "help" by shifting the lid a little. This looks frightening and if you are 6 years old, it IS frightening so quickly drop the lid! Mom only yells for a second--the blood spurting from her arm surprises everyone!
This is much more effective than trying to find a crack in a brand-new sidewalk.