Actually, I don't think I've ever heard my granddaughters use that phrase; they're much more direct: they say, "I'll be the mommy" or "I'll be the baby".
Because they're not pretending, they're being.
But only briefly. The thing is to be "mommy", to feel oneself in that role- not to stay in it and be tied down to doing mommyish things- because that would soon become boring.
Their games are free form. A situation is imagined, briefly inhabited, then another takes its place. There's very little narrative. Time hardly exists. Their play is a kind of dreaming- disjointed, fragmented, but intensely felt. Things matter terribly and then, just like that, cease to matter because something else has come along.
Sometimes they are happy to imagine the same thing, sometimes their wills diverge- and then they bicker- but the bickering soon dies down. Adult intervention is rarely required.
This morning they were playing with the doll's house. They asked me to join in, but once they got deep into the play I was no longer needed. Some dogs (pottery ornaments) were introduced. I suggested a visit to the pet shop. This lasted for a minute or so- then morphed into constructing a living space for the dogs, using the doll's house furniture. Creating environments consumes more time and is more absorbing than doing things with the environment once it's been set up.
There's no morality. You don't need morality when there's no narrative. Things happen in a void- with no cause and no consequence. Granny and Grandpa doll come to visit their family but there's no place for them in the current game so they're pronounced dead on arrival, trampled under foot by their offspring- and cast aside. Later the boy doll is thrown across the patio because why not? No-one is really hurt. No-one really dies...