I’ve got my 12 week scan tomorrow and if everything looks okay, we are going to tell my 4 year old…

I’ve got my 12 week scan tomorrow and if everything looks okay, we are going to tell my 4 year old daughter tomorrow. We’ve got her a T-shirt saying Big Sister with her name and Minnie Mouse on just to wear when she wants to s she can’t really read. Any ideas on how we can tell her the news ?