You will get plenty of advice on here but I would say a lot of digging out in the line of the hedge, Mkm.( until there is no sign of the couch grass left,I should imagine the gorse will be easier,) go down at least two spades depth and in-fill with good compost & manure etc. That is the hard part over, the clearing, the best bit will be the planting with the children, how lovely to encourage them, have fun.