|Spellings vary . . .|
|I prefer the authentic Italian orthography.|
Why does the /p/ change into a /b/? Well, in fact it doesn't change at all -- but the rules of English phonetics mean that the same sound -- an unaspirated voiceless bilabial stop -- is heard as /p/ when it comes after /s/ but as /b/ when it begins a word. The so-called "voiced" (lenis) stops /b d g/ are actually voiceless in word-initial position, and are distinguishable from their "voiceless" (fortis) counterparts /p t k/ because the latter are aspirated. However, fortis stops lose their aspiration after /s/, so the distinction between the two is lost in that position. That's why disgust and discussed are homophones for most speakers, and why it's so easy to mishear Hendrix's "kiss the sky" as "kiss this guy." Sky is realized as [skaɪ], so when the /s/ is removed, it leaves guy [kaɪ] (not kie [kʰaɪ]). Thus, when the /s/ in spaghetti is moved, both the /p/ and the /g/ change their character, being heard as different phonemes in spite of not actually changing their sound. Hence baschetti.
The question is, why do children only do this with the word spaghetti? It's common for children to simplify consonant clusters by simply omitting one of the consonants (as in myyo for smile), but spaghetti is the only word I'm aware of in which the /s/ is not omitted but transposed to a different part of the word. We might expect, by analogy with baschetti, that some children would pronounce spider as byster, for example, but they don't, as far as I know. Byder, yes; byster, no. Nor have I ever heard of a deskosaurus.
My best guess is that it has to do with spaghetti being stressed on the second syllable, and that the /s/ is being moved from an unstressed syllable to a stressed one. I can't think of any other common words where the first syllable is unstressed and begins with /s/ + a fortis stop, and the second syllable is stressed and begins with a lenis stop, which could explain why baschetti is the only one I've ever heard. Few little kids are likely to have words like stability and twenty-three skidoo in their vocabulary, so the hypothesis that they might pronounce them as daspility and twenny-fwee gistoo never gets to be tested.
Update: Here are some stats on the relative frequency (measured in Google hits) of various spellings of this mispronunciation.
- pasghetti (32,900)
- pasgetti (21,700)
- bisgetti (20,700)
- pisgetti (17,800)
- pasketti (16,400)
- basghetti (14,000)
- basketti (9,260)
- bisghetti (8,660)
- basgetti (6,690)
- pisketti (4,240)
That's a total of 152,350 hits, which can be analyzed as follows.
- 61% begin with "p"; 39% begin with "b"
- 66% have "a" as the first vowel; 34% have "i"
- 44% have "g"; 36% have "gh"; 20% have "k"
The following spellings were not included in the analysis because most of the Google image search hits for them were not pictures of spaghetti. ("Pisghetti" arguably should have been included, since it mostly yielded pictures of a Curious George character called Chef Pisghetti, who is obviously named after the mispronounced pasta dish.)
- baschetti (252,000)
- bischetti (154,000)
- pisghetti (19,300)
- bisketti (17,800)
- paschetti (15,600)
- pischetti (2,790)