Take a spotlessly clean bucket, into which you place 4 heads of Elderflower (ideally the flowers should be fully open, have had some sun on them, and be picked before mid-day), the zest of 1 lemon (no white pith), the juice of one lemon, 1 and a half pounds of sugar, and 2 tablespoons of white wine vinager (or any other that's colourless) . Into this pour 6 litres of water and stir until all the sugar has disolved. Cover with a clean cloth and wait for 24 hrs.
The following day, strain the liquid through muslin as it's poured into the bottles, and leave for 2 weeks before drinking. The resulting 'Champagne' should be extremely effervescent, so always open outdoors. Serve chilled. It's as easy as that!
On the thorny question of alcohol, I suppose there must be a tiny amount in it, but really not enough to deprive children of the fruits of their labour. Just don't give them too much.