United Kingdom, Jan. 21 -- Meghan Trainor's sons are thrilled to have a sister.

The 32-year-old singer and her 33-year-old husband, actor Daryl Sabara, welcomed baby Mikey Moon into the world via a surrogate on January 18 - but shared the joyous news via Instagram on Tuesday (20.01.26) - and Meghan adores seeing their boys, Riley, four, and Barry, two, get to know her.

She told People: "They are over the moon. Riley takes his 'big brother' role very seriously, and Barry is just so gentle and curious with her. Watching them bond with their sister has been one of the sweetest parts of this whole experience."

And Riley - who, with Barry, chose their new sibling's name - cannot wait to tell his school class about the new addition to his fa...