@JustWilliam
The baby bottle with a tasteless laxative, can be fun. Of course you must have their prior consent else you may have problems.
That said, the ‘baby’ scene can be played several ways: They can be told the bottle has a strong laxative either before or after they have drunk it or not at all. The first two options are a mind trip, be cause the baby will know to expect something to happen, but not what or when. If the baby is not told, they will get a big surprise when the laxative starts working and they lose control, and they will lose control.
The Sorbitol, Latulose, or Picoprep will usually cause a very sloppy, messy poo, usually within 6 hours or less. The results will be messy and incontinent pants over the diaper will be needed. There will be a lot of cleanup.
If you have more play time, a dose of Miralax accompanied by a dose of Senna is preferred if you want a soft, urgent, but not sloppy, poo. The Miralax dose can be adjusted to get the desired softness. This combination is also preferred if the baby like resisting pooing. Depending on the Senna dosage, resistance is usually possible for an hour or more. The Senna can be given baked into a brownie, the Miralax in lemonade. This combination is unlikely to do anything for at least 8 hours, and may take over 12. This combination is more fun if the baby is somewhat constipated at the outset.