On 19 June, 1221, Margaret married Hubert de Burgh. At the time of their marriage Hubert was effectively the Regent of the Kingdom of England since Henry III was an underage King. Henry III finally came of age in 1227 and Hubert retired from his duties as Regent. He was awarded the title of Earl of Kent and remained one of the most influential people at court. With Margaret as his countess.
They had only one known daughter:
From her birth to her death was Margaret was arguably either first or second heir to the throne of the Kingdom of Scotland as one of the few living, legitimate descedants of William I. However it should be noted that cognatic primogeniture was not yet the norm in Great Britain and more distant relatives could well claim the throne. As they would in fact do in the Succession Crisis of 1290 to 1292.