I'm pretty sure giggle does have childish connotations (it's associated with children), but my question is really; is it so associated with children that it would seem weird to a native speaker if giggle was used to describe an adult's laugh.
Allison giggled, "Definitely. It's very good."
Context: Someone just described a book in a somewhat funny way; there's romantic tension here.
So, does this word-choice feel jarring, considering Allison is a young adult?
In my experience, giggling is a normal word to use for a particular kind of laughter (usually an expression of silliness and not very loud) exhibited by a person of any age. It does not have any age-specific connotations on its own. Children giggle more often than adults, but not overwhelmingly so.
I'm pretty sure giggle does have childish connotations
This is your problem. You will not find any dictionary to agree with you.
OED:
a. intransitive. To laugh continuously in a manner not uproarious, but suggestive either of foolish levity or uncontrollable amusement. Cf. snigger v.1, titter v.2
1509 A. Barclay Brant's Shyp of Folys (Pynson) f. xxxv Some gygyll and lawgh without grauyte.
1851 D. Jerrold St. Giles & St. James (new ed.) xv, in Writings I. 154 All men in the court laughed, and the pretty ladies giggled.
1876 L. Stephen Hours in Libr. 2nd Ser. iv. 182 The striking scene..when the House of Commons was giggling over some delicious story of bribery and corruption.
The 1851 quote is illustrative: giggling can be done self-consciously (as if aware that one should not be laughing) and Victorian women were famous for being shy and ladylike.
A giggle is a cheerful silly laugh. When socially appropriate it is normally seen as charming, not jarring. In the 2020’s America where any gender specific connotation is jarring, all bets are off.
Giggling can be not only adult but actually frightening in some circumstances. Ian Fleming created the sadisitic character Tee Hee Johnson in the book Live and Let Die. The character giggled in anticipation of the pleasure he would gain from the inflicting pain. He was introduced in the following passage:
'Tee-Hee, break the little finger of Mr. Bond's left hand.'
The negro showed the reason for his nickname.
'Hee-hee,' he gave a falsetto giggle. 'Hee-hee.'
He walked jauntily over to Bond. Bond clutched madly at the arms of his chair. Sweat started to break out on his forehead. He tried to imagine the pain so that he could control it.
The negro slowly unhinged the little finger of Bond's left hand, immovably bound to the arm of his chair.
The giggle contributed greatly to the horror generated by the character.
