Help! Any fun ideas how to tell neice & nephew

Has anyone told nieces and nephews that are young (2 yrs & 5 yrs) they are going to be big cousins?! - I know our 2 ye old niece won’t understand she is going to be a big cousin but our 5 yr old nephew definitely will! I have been trying to think of a fun way to tell them but there are no good/fun books out there that I could find and being the holidays shirts would take over a month to ship/arrive! Was hoping for any fun ideas of something I could put together myself for them!?