First we need to know that it takes roughly one litre of helium to lift one gram so to lift a child weighting 25kg we would need 25,000 litres of helium, that's quite a lot. If we assume that an average helium filled balloon is 25cm in diameter we can use the equation 4/3 x pi x r x r x r to find the volume.
By taking pi to 2 decimal places and the radius as 12.5cm we get:
4/3 x pi x 12.5 x 12.5 x 12.5 = 8,177.08 cubic centimetres, which is just over 8 litres.
If we assume that the balloon and the string weight 2 grams, each balloon could lift just 6 grams in addition to its own weight. For your child that weights 25kg you'd need 4,167 balloons and roughly £8,500 in cash to lift him or her off the ground, probably not something anyone will do by accident.