I think the milk was drugged so they could go out without her. Sini miscalculated the amount and she slipped into an unconscious state while they were at dinner and they couldn't explain that, so . . .they let her die without calling 911.
I think they used the spoons to keep the doors of the kitchen or cupboards closed with the child locked inside.
Just read a story where 3 adults gave a minor, challenged, 14 year old child blood pressure medication for a few weeks which caused her death. It made her sleepy and easier to handle. Of course, she wasn't prescribed this medication. I think Sini used her knowledge of meds to drug Sherin so they wouldn't have to put up with her in public.
No one could make me leave a 3 year old home alone unless I was kidnapped. So, Sini was in on the "Sherin-free" dining experience which makes her a very negligent and selfish person.
WM has already put Sini in the hot seat, which would anger Sini - a betrayal - so I think they will start to tell on each other in attempts to save themselves more severe punishment.
Most importantly, they didn't order a take home meal for Sherin, so they had no intention of feeding her when they returned. Why would you not bring food home for the daughter you left behind? Were they expecting her to be knocked out from the drugs and therefore unable to eat?