如何用英文祝贺小孩（被祝贺人的孙辈）降生

I am really really glad to hear that you got a new XXX(family name) in your family.I can't wait to see this little angel.It must be beautiful because it's parents are so amazing.I am sure of that it will be the most brilliant kid,and you will be the most perfect parents.
I am happy for you(如果关系好的话可以说I am so proud of you)

best wishes.God bless you

Congratulations!
Best wishes to the baby and your family!