The “Pretender in Spain” is a reference to James Francis Edward Stuart, who made a claim to the British throne in 1701. He was called "The Old Pretender" because, though he was son of King James II, he was a Catholic and, in 1701, the Act of Settlement was passed making it illegal to for the throne to be held by any non-protestant. He was recognized, however, as the King of England, Scotland and Ireland by the Pope and the King of France and he ultimately lived out his days in Rome where he established a court in exile. Most of the Irish were Catholic and viewed the English as oppressors. He was, therefore, a symbol of hope to the Irish and some left Ireland to enlist in support of him. More info here. Swift is suggesting that this large cohort of poor, Irish children are likely to be supporters of a figure who is a threat to the English monarch.