Why do you think there was a painful pause when Winnie's family realizes she actually went with the Tucks on her own free will in Tuck Everlasting?

This question deals with information that is given to readers in the early parts of chapter 21. The constable had just brought Winnie home, and the Foster family is fawning over her return. The constable informs the family that Winnie had gone with the Tucks by her own choosing, and the comment causes a "painful pause" that lasts only for a moment.
Readers are not told why this pause exists, so it is up to individual readers to come up with a possible reason. One possible reason is that they are simply too shocked at the information to form a coherent response. The "painful" part of the pause is the part that needs further probing. Simply being shocked into silence makes sense. Having it be painful is a different matter.
The Foster family is a stifling family. They hover over Winnie's every move. It is why she longed to run away at the beginning of the story:

"See?" said Winnie to the toad. "That's just what I mean. It's like that every minute. If I had a sister or a brother, there'd be someone else for them to watch. But, as it is, there's only me. I'm tired of being looked at all the time. I want to be by myself for a change."

I think one reason the Fosters are painfully shocked into silence could be because they simply can't fathom their daughter actually wanting to go with any other family. Another possibility is that they are shocked into painful silence because, just for a brief moment, they realize that their hovering parenting style is what drove Winnie to run away with the Tucks in the first place.