Their names are Mashed Potato and Green Bean. Morgan is a grown-up, and specifies this in every story. Mashed Potato is anywhere between 3 and 5 1/2 years old, and Green Bean is 1 to 2 1/2. (The difference between the ages is not consistent.) Morgan informs me that "They're not boys, they're just babies!" but consistently uses masculine pronouns for both of them.
She likes to use them to work out things that have just happened to her, where she represents me and Mashed Potato represents her. So when she got a pain in her foot and needed to be carried for the rest of the dog walk, the Morgan's Babies stories that day and the next were about how Mashed Potato stepped on something sharp, so Morgan took his shoe off and cleaned off his foot, and then carried him, and I carried Green Bean, because Green Bean is too little to walk the whole way.
This morning they all went to a gym class, but first Morgan made octopus cupcakes for Mashed Potato's birthday ("It not his birthday yet, but I have to be prepared!") and cut out a red felt heart and put it up in his room with sticky-tack.
I am fascinated to see how this develops.