Winnie makes a difference in the lives of several characters in this great book. I think she makes a big difference in the life of Mae Tuck. If Winnie had not come into her household, Mae likely wouldn't have hit and killed the man in the yellow suit. She wouldn't have been thrown in jail and sentenced to hang, either. Winnie tries to make up for that by helping Mae escape from jail. Winnie takes Mae's place to fool the constable, and that gives the Tucks more time to escape Treegap. To be honest, the Tuck men would have gotten Mae out of jail with or without Winnie's help.
The other person that Winnie very much made a difference for is Jesse Tuck. Jesse Tuck is so smitten with Winnie that he openly admits that he would consider spending eternity with her.
We could get married, even. That'd be pretty good, wouldn't it! We could have a grand old time, go all around the world, see everything.
Jesse is notoriously spontaneous, yet he is willing to tie himself to a single person forever. That's a huge impact that Winnie has made on his life. While it does come across as cute, I do like to remind readers that Jesse is an eternal seventeen-year-old, and Winnie is ten. That's a high school junior asking a fourth-grader to marry him some day.
Finally, Winnie makes a huge difference in the life of the toad. She gives it the spring water and the gift of eternal life.