I did this but honestly in hindsight it didn’t tell me a lot. Two reasons come to mind:

1. Watching other peoples’ kids is a time-limited exercise. A major challenge with your own kids is that it’s constant and effectively without end.

2. There’s a big difference between kids and MY kids. I just have more intrinsic interest in my own. I do have more “tools” available to me to engage with others’ kids now (up to my own kids’ ages at least) but I care less. Not saying I dislike others’ kids, just that it’s different.

I remember having my friends (with two kids, 2 and 5) visit us before we had our own children. We have spent a full day together. I was so exhausted with the amount of stimuli that I thought I'll never have my own children. Yet, two years later our first kid was born.. and still, the amount of stimuli is sometimes above my norm. Such is life :)