What do you think Aunt Polly's reaction is when she finds out about Tom's exploits?

Aunt Polly's ambivalence about Tom is clear from the very beginning of the book: she is infuriated and concerned by his misbehavior, but at the same time she feels very tenderly toward him. We see this right away in chapter 1, where the very first scene in the book concludes with something of a soliloquy by Aunt Polly in which she says:

"He ’pears to know just how long he can torment me before I get my dander up, and he knows if he can make out to put me off for a minute or make me laugh, it’s all down again and I can’t hit him a lick. I ain’t doing my duty by that boy, and that’s the Lord’s truth, goodness knows. Spare the rod and spile the child, as the Good Book says. . . . He’s full of the Old Scratch, but laws-a-me! he’s my own dead sister’s boy, poor thing, and I ain’t got the heart to lash him, somehow. Every time I let him off, my conscience does hurt me so, and every time I hit him my old heart most breaks." (emphasis added)

The last sentence, especially with the balanced repetition of its phrasing, perfectly captures the two contrary responses Tom provokes in Aunt Polly.
This contradiction is shown again in her reaction to what is probably Tom's most outrageous prank: pretending to be dead only to return, together with Joe and Huck, at his own funeral. At the very end of chapter 16, after the boys return, we are told that

Tom got more cuffs and kisses that day—according to Aunt Polly’s varying moods—than he had earned before in a year.

These actions reflect exactly what Aunt Polly's words expressed in chapter 1: she is equally inclined to shower him with discipline or affection.
As chapter 17 opens, however, we seem to see a possible resolution of these contrary reactions. The very next day after all the "cuffs and kisses," Aunt Polly seems to have nothing but tenderness toward Tom:

At breakfast, Monday morning, Aunt Polly and Mary were very loving to Tom, and very attentive to his wants.

This is confirmed after Tom pretends to have had a prophetic dream of a night during his absence (in reality, he was watching through the window) which concluded with him giving her a kiss and leaving a note reassuring her that he was alive. Aunt Polly's response is entirely affectionate in both word and deed:

“Did you, Tom, did you! I just forgive you everything for that!” And she seized the boy in a crushing embrace.

Even when Aunt Polly finds out in chapter 19 that Tom lied about the dream, she says, " 'Tom, I've a notion to skin you alive!' " but doesn't make a move. When Tom admits that he was really there but still maintains that he gave her a kiss and wanted to leave the note, she supposes this to be part of the lie but is still entirely willing to forgive him:

"I reckon he’s lied about it—but it’s a blessed, blessed lie, there’s such a comfort come from it. I hope the Lord—I know the Lord will forgive him, because it was such good-heartedness in him to tell it."

Finally discovering that he was telling the truth about the note, she is overcome:

A moment later she was reading Tom’s piece of bark through flowing tears and saying: “I could forgive the boy, now, if he’d committed a million sins!”

Clearly, there is little left of her earlier desire to discipline him physically.
So, when we are told in chapter 32 only that "Aunt Polly’s happiness was complete," there can be little doubt that her response is entirely one of tender, and probably tearful, welcome, with hardly any trace of her earlier irritation.