this is going to be rad, film it as they open the door
Yup thats the plan
Have you timed it? How long to fill and tie one balloon?
Consider using cable ties. Or something just as quick.
Ye should be sorting something along the lines of this
http://lifehacker.com/5824498/tie-k...loads-water-balloons-so-easily-kids-can-do-it
), pushed it through his letterbox and inflated it with a bike pump in the dead of night. His mum was most displeased 


