We put our 2 1/2 year old in a 'big brother' t-shirt and then just waited for Grandma to notice. She was totally excited, her mouth fell open, and all she could say was "really?!?!? really?!?!" then raced across the kitchen and practically tackled me, trying to hug me.
Yes, I know, not too creative, but certainly effective!
(For the first kid, we put the positive pregnancy test in a plastic baggy then wrapped that. When they went to open it, they had no clue what it was, started to open the plastic baggy when we had to call out "Oh no!" and then had to explain it to them)
Edited to add - Michelle P's idea sounds really cool!