It’s hard to tell in this video if Mackenzie is smiling at the music and toys of the playmat or at her big sister Maddie. I’d like to think it’s Maddie who’s making her smile. 
This is a little long, but if you make it to the end, you’ll hear Mackenzie make a few little cooing sounds.
She is so adorable! and that is my unbiased opinion.
My biased opinion includes many more complimentary adjectives.
Love,
those smiles are the greatest! I’m jealous that you were able to document it on camera…. A great thing I have yet to achieve.