The thing that has changed the most for me since I was a kid is ....
この場合、次のように言えますよ。
ーThe thing that has changed the most for me since I was a kid is ....
「私が子供の頃からもっとも変わったことは…です」
例:
A: What has changed the most for you since you were a kid?
「あなたが子供の頃と比べて一番変わったことはなんですか?」
B: I notice the summer heat now much more than I did when I was a kid.
「子供の頃より夏の暑さが厳しくなったことです」
ご参考まで!