The following was on a site where they describe how to modify a kite for fishing with. The idea is that the balloons filled with air, make the kite self launching if it lands in the water. But if it were filled with helium it would also presumably stabilise a kite & keeps it airborne for longer.
The long skinny balloons they make animals and things out of are the easiest way of providing flotation along the centre of any delta kite.
A. Tie the ends of the balloons together
B. Tie a knot in each balloon 5 or 7cm from the first knot
C. Blow the balloons up and tie them off leaving a few centimetres of tail
D. Tie the two C ends together with a knot that will not slip
E. Pull end A over the nose of the kite and the D end over the bottom of the centre spar
If the balloons are too short for your kite when stretched tie a length of fishing line or dacron cord to the A end and fix to the nose of the kite.
Finally pull the keel of the kite through between the two balloons.
The reason the part of the balloon in front of the keel is not inflated is that it may get caught in the wind and cause the kite to nosedive.