"poo poo"

[sagehen]

Rather than having any particular meaning it seems likely to me that "pooh
pooh" originally was simply a conventionalized spelling  of a non-verbal
dismissive sound (unvocalized plosive). Just as "ha ha," "ho ho," "heh
heh," &c., were   conventions for conveying the non-verbal sounds of
laughter, which then took on  life as words, "pooh pooh" has become a verb
meaning /belittle/.
The coincidence of its (sometimes) meaning /bullshit/ with the infantile
"poo" 's meaning /shit/, IMO is merely fortuitous.
A. Murie