The physical cost is inconsequential. The risk to Biyu's health isn't.
Lets not forget how painful the first breakthrough actually is. Hell, all cultivation is hard and/or boring, not something that'll keep the attention of a 3-4 years old.
Whereas Biyu seeing her mom "play with the shinny stone" every (couple?) day is probably going to keep her interested. Specially if mom takes the time to talk to Biyu about it.
I don't remember anything about first breakthrough being painful, just .Meizhen's fast-track method of helping Ling Qi find her Dantian.
Though maybe it's not possible for a child to start cultivating without that pain.