This struck me as a pretty interesting utterance.
Here's the relevant background information. First, the Standard English past tense here is formed by two simple rules.
One more piece of background information that is important is to understand Grace's rules for syllabification. Grace has for a long time not permitted consonant clusters. This is pretty typical of children's speech in English, and is typical of many languages throughout the world so it's also true of many accents. Her basic rule is that a syllable consists of C + V + C, where C is one consonant, V is one or more vowels and C is one consonant. When a word contains more than one consonant, Grace has two options: she can ignore a consonant or add a vowel. In practice, she does both, as in the following pronunciations:
1. Gramatical rule: Add "d" to create the past tense (unless the word already ends in "d/t" in which case you add "ɪd" in order to make the "d" audible).
2. Phonological rule: In a consonant cluster, the voicing spreads from the first consonant rightward in English (the opposite can happen at other times -- as it does in Spanish, for example, where voicing spreads leftward to affect the pronunciation of the "s" in "mismo" or "beisbol").
poop --> (grammar) poopd --> (phonology) poopt
Grace's syllabification at work
James: "dʒeimɪz" (standard pronunciation: dʒeimz)
Books: "bʊtʰɪz" (standard prononciation: bʊks)
Star: "tʰɑɹ" (standard pronunciation: stɑɹ)
These have been standard Gracese for a while. Obviously under the pressure of the Standard English she hears every day, all of these formations will eventually give way to the standard ones, and we have started to hear the occasional consonant cluster out of Grace already.
Analysis of Poop-it
Given Grace's rules for syllables, and Grace's frequent formation of past tense words in English with the "d" suffix, I would have expected her to form one of the following two forms:
1. Gracese: pʰupʰɪd (poop-id)
2. Standard: pʰupɾ (poopt)
What she did, however, is she formed a hybrid. This raises the possibility that she has learned neither the grammar nor the phonological rule described above, but has instead simply heard the word form "poopt" and interpreted it based on her own rules for syllabification.
This shouldn't seem that surprising, but to me it is. Listening to her talk, it seems so clear that she is using our grammatical endings "z" (plural, third person) and "d" (past tense) in a variety of contexts. But of course, it's entirely possible that instead of hearing Grace alternate between uninflected forms ("I poop") and inflected forms ("I poopit"), what I have in fact been hearing is simply her alternating between two pronunciations/interpretations of the to-her unpronouncable ("I poopt): "I poop" and "I poopit").
Of course the real proof that she is inflected verbs at all will only come when she begins to create novel (i.e. erroneous) forms like "goed" and "throwed." I can't say for sure whether I've heard these yet -- given how common they are in kid-speak and how interested I am, it's likely we haven't hit this phase yet.
Except that my second example of Grace's syllabification above is in fact a novel form: bʊkɪz. If Grace were in fact simply hearing and mimicking the word form, we would predict "bʊk" or "bʊkɪs". So are these novel forms? Has Grace learned to inflect for number but learned tense simply as "vocabulary"?
Hard to know for sure, but for the next few days I'll be paying close attention to those verbs and nouns that end in unvoiced consonants.