The music mogul's mansion in Holland Park, west London, was entered in the early hours of Friday morning, with an intruder taking cash and jewellery.
Cowell told The Sun newspaper that he and partner Lauren Silverman ran to check on the 21-month-old child after a bodyguard woke them to tell them about the break-in.
He referred to the incident after the first act performed on the semi-final of the ITV show.
When he was asked what he thought of their performance, Cowell said: "What was that about?"
He then joked: "Very appropriate the night after I get burgled."