Maybe I'm wrong, but I think since they're in there together, and Macy isn't a speechless baby, they're fine with the door open while Dani goes and grabs some things.
My mama used to do the same thing with us.
Liam is also barely younger than Macy.
It's not like they're infants.
I am sorry if you take my worry as an insult but children do not always fully comprehend situations. I would not trust ANY 3 year old to tell the difference between her brother playing in the water and drowning.
I am allowed to have my concerns and I am not bashing you at all so any problem you have with it is yours personally.