Well I'm not exactly sure how to get a pochitchi but you get a pochitchi on a odd generation and I think you have to treat your tama good but not great. And the only teenager I know of that can turn into a Pochitchi is a Oniontchi. HOPE THIS HELPS!!!
Well I don't knowhow your suppose to treat them but it seems you can get the oniontchi from all 4 of the toddlers. The kinatomotchi, the marutchi, the hitodetchi and the Kuribotchi