This is cute and touching. I'm jealous. I miss it when my daughters were this age. Awesome work dad.
The entire period from 3 months to around 5-6 is magical - they are seeing new things, trying new things, learning at a voracious rate, and constantly surprising you. They are still great after that but they are more on their path to who they will become, and mom and dad of course become a little less cool.
Mine are grown and are wonderful people, but I can't tell you what I would give to go back and spend a day at the park with them at 3-4 years old.
We had, on advice from a nurse, tried to rule out milk protein earlier by mother not drinking any milk. She did however not say that you have to do that for a very long time before you can see any results. Think it was at least three-four weeks until you could say if it worked or not.
Luckily the milk protein allergy grows away before 7 years old and she is perfectly fine with milk now.