An 8-year-old child complains of a painless round formation under the tongue on the left which was noticed by accident about a month ago. Objectively: there is a roundish formation up to 2,0 cm in diameter in the sublingual region deep in the soft tissues of mouth floor. It is soft, painless to palpation, the mucous membrane over it is of grayish-blue color. What is the most likely diagnosis?