The pontiff turns 77 on Tuesday. The children presented the treat after the pope visited a Vatican dispensary which provides paediatric care.
Francis blew out the candles, thanked the youngsters for the cake and promised: "I'll tell you later if it's good or not."
At one point, Francis briefly lost his white skull cap to a child he was holding. The pope smiled and put it back on his head.
For more than 90 years, the dispensary on the Vatican grounds has been distributing milk, clothing, nappies, toys and even baby carriages to families in need.
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